Support Flow's proto modifier syntax for declared classes (babel#7978)

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| Minor: New Feature?      | Yes
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| License                  | MIT

See facebook/flow@eb815be for more information about this feature.
    
The proto modifier indicates that a property declared using `x: T` syntax is actually present on the prototype object of the class, rather than an own property.
    
The proto and static modifiers are mutually exclusive, as class declarations don't simultaneously define the static prototype object, as they do the instance prototype.
    
This syntax is only supported on declared classes, not object types, interfaces, or runtime class declarations, and as such should only appear in library definitions.

src/plugins/flow.js

@@ -427,7 +427,10 @@ export default (superClass: Class<Parser>): Class<Parser> =>
 
     // Interfaces
 
-    flowParseInterfaceish(node: N.FlowDeclare, isClass?: boolean): void {
+    flowParseInterfaceish(
+      node: N.FlowDeclare,
+      isClass?: boolean = false,
+    ): void {
       node.id = this.flowParseRestrictedIdentifier(/*liberal*/ !isClass);
 
       if (this.isRelational("<")) {
@@ -460,7 +463,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         } while (this.eat(tt.comma));
       }
 
-      node.body = this.flowParseObjectType(true, false, false);
+      node.body = this.flowParseObjectType(true, false, false, isClass);
     }
 
     flowParseInterfaceExtends(): N.FlowInterfaceExtends {
@@ -653,7 +656,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         } while (this.eat(tt.comma));
       }
 
-      node.body = this.flowParseObjectType(true, false, false);
+      node.body = this.flowParseObjectType(true, false, false, false);
 
       return this.finishNode(node, "InterfaceTypeAnnotation");
     }
@@ -755,6 +758,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       allowStatic: boolean,
       allowExact: boolean,
       allowSpread: boolean,
+      allowProto: boolean,
     ): N.FlowObjectTypeAnnotation {
       const oldInType = this.state.inType;
       this.state.inType = true;
@@ -782,8 +786,19 @@ export default (superClass: Class<Parser>): Class<Parser> =>
 
       while (!this.match(endDelim)) {
         let isStatic = false;
+        let protoStart: ?number = null;
         const node = this.startNode();
 
+        if (allowProto && this.isContextual("proto")) {
+          const lookahead = this.lookahead();
+
+          if (lookahead.type !== tt.colon && lookahead.type !== tt.question) {
+            this.next();
+            protoStart = this.state.start;
+            allowStatic = false;
+          }
+        }
+
         if (allowStatic && this.isContextual("static")) {
           const lookahead = this.lookahead();
 
@@ -797,6 +812,9 @@ export default (superClass: Class<Parser>): Class<Parser> =>
         const variance = this.flowParseVariance();
 
         if (this.eat(tt.bracketL)) {
+          if (protoStart != null) {
+            this.unexpected(protoStart);
+          }
           if (this.eat(tt.bracketL)) {
             if (variance) {
               this.unexpected(variance.start);
@@ -810,6 +828,9 @@ export default (superClass: Class<Parser>): Class<Parser> =>
             );
           }
         } else if (this.match(tt.parenL) || this.isRelational("<")) {
+          if (protoStart != null) {
+            this.unexpected(protoStart);
+          }
           if (variance) {
             this.unexpected(variance.start);
           }
@@ -835,6 +856,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
             this.flowParseObjectTypeProperty(
               node,
               isStatic,
+              protoStart,
               variance,
               kind,
               allowSpread,
@@ -857,6 +879,7 @@ export default (superClass: Class<Parser>): Class<Parser> =>
     flowParseObjectTypeProperty(
       node: N.FlowObjectTypeProperty | N.FlowObjectTypeSpreadProperty,
       isStatic: boolean,
+      protoStart: ?number,
       variance: ?N.FlowVariance,
       kind: string,
       allowSpread: boolean,
@@ -868,6 +891,9 @@ export default (superClass: Class<Parser>): Class<Parser> =>
             "Spread operator cannot appear in class or interface definitions",
           );
         }
+        if (protoStart != null) {
+          this.unexpected(protoStart);
+        }
         if (variance) {
           this.unexpected(
             variance.start,
@@ -881,13 +907,17 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       } else {
         node.key = this.flowParseObjectPropertyKey();
         node.static = isStatic;
+        node.proto = protoStart != null;
         node.kind = kind;
 
         let optional = false;
         if (this.isRelational("<") || this.match(tt.parenL)) {
           // This is a method property
           node.method = true;
 
+          if (protoStart != null) {
+            this.unexpected(protoStart);
+          }
           if (variance) {
             this.unexpected(variance.start);
           }
@@ -1116,10 +1146,10 @@ export default (superClass: Class<Parser>): Class<Parser> =>
           );
 
         case tt.braceL:
-          return this.flowParseObjectType(false, false, true);
+          return this.flowParseObjectType(false, false, true, false);
 
         case tt.braceBarL:
-          return this.flowParseObjectType(false, true, true);
+          return this.flowParseObjectType(false, true, true, false);
 
         case tt.bracketL:
           return this.flowParseTupleType();

test/fixtures/flow/call-properties/3/output.json

@@ -284,6 +284,7 @@
                         "name": "y"
                       },
                       "static": false,
+                      "proto": false,
                       "kind": "init",
                       "method": false,
                       "value": {

test/fixtures/flow/class-properties/getter-setter/output.json

@@ -112,6 +112,7 @@
                 "name": "a"
               },
               "static": false,
+              "proto": false,
               "kind": "get",
               "method": true,
               "value": {
@@ -181,6 +182,7 @@
                 "name": "b"
               },
               "static": false,
+              "proto": false,
               "kind": "set",
               "method": true,
               "value": {
@@ -302,6 +304,7 @@
                 "value": "c"
               },
               "static": false,
+              "proto": false,
               "kind": "get",
               "method": true,
               "value": {
@@ -374,6 +377,7 @@
                 "value": "d"
               },
               "static": false,
+              "proto": false,
               "kind": "set",
               "method": true,
               "value": {
@@ -495,6 +499,7 @@
                 "value": 1
               },
               "static": false,
+              "proto": false,
               "kind": "get",
               "method": true,
               "value": {
@@ -567,6 +572,7 @@
                 "value": 2
               },
               "static": false,
+              "proto": false,
               "kind": "set",
               "method": true,
               "value": {

test/fixtures/flow/class-properties/named-static/output.json

@@ -112,6 +112,7 @@
                 "name": "static"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/comment/02-type-include/output.json

@@ -109,6 +109,7 @@
                 "name": "foo"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -161,6 +162,7 @@
                 "name": "bar"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -213,6 +215,7 @@
                 "name": "baz"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/comment/03-type-flow-include/output.json

@@ -109,6 +109,7 @@
                 "name": "foo"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -161,6 +162,7 @@
                 "name": "bar"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -213,6 +215,7 @@
                 "name": "baz"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/declare-export/export-class/output.json

@@ -175,6 +175,7 @@
                         "name": "meth"
                       },
                       "static": false,
+                      "proto": false,
                       "kind": "init",
                       "method": true,
                       "value": {

test/fixtures/flow/declare-module/10/output.json

@@ -105,6 +105,7 @@
                   "name": "foo"
                 },
                 "static": false,
+                "proto": false,
                 "kind": "init",
                 "method": true,
                 "value": {

test/fixtures/flow/declare-module/5/output.json

@@ -158,6 +158,7 @@
                       "name": "foo"
                     },
                     "static": false,
+                    "proto": false,
                     "kind": "init",
                     "method": true,
                     "value": {

test/fixtures/flow/declare-module/6/output.json

@@ -151,6 +151,7 @@
                         "name": "foo"
                       },
                       "static": false,
+                      "proto": false,
                       "kind": "init",
                       "method": true,
                       "value": {

test/fixtures/flow/declare-statements/10/output.json

@@ -112,6 +112,7 @@
                 "name": "foo"
               },
               "static": true,
+              "proto": false,
               "kind": "init",
               "method": true,
               "value": {
@@ -181,6 +182,7 @@
                 "name": "x"
               },
               "static": true,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/declare-statements/15/output.json

@@ -112,6 +112,7 @@
                 "name": "foo"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -254,6 +255,7 @@
                 "name": "foo"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/declare-statements/17/output.json

@@ -112,6 +112,7 @@
                 "name": "a"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -164,6 +165,7 @@
                 "name": "b"
               },
               "static": true,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -216,6 +218,7 @@
                 "name": "c"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/declare-statements/7/output.json

@@ -112,6 +112,7 @@
                 "name": "didAnimate"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": true,
               "value": {

test/fixtures/flow/declare-statements/9/output.json

@@ -228,6 +228,7 @@
                 "name": "x"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/interface-types/basic/output.json

@@ -124,6 +124,7 @@
                   "name": "p"
                 },
                 "static": false,
+                "proto": false,
                 "kind": "init",
                 "method": false,
                 "value": {

test/fixtures/flow/interface-types/extends-multiple/output.json

@@ -191,6 +191,7 @@
                   "name": "p"
                 },
                 "static": false,
+                "proto": false,
                 "kind": "init",
                 "method": false,
                 "value": {

test/fixtures/flow/interface-types/extends/output.json

@@ -158,6 +158,7 @@
                   "name": "p"
                 },
                 "static": false,
+                "proto": false,
                 "kind": "init",
                 "method": false,
                 "value": {

test/fixtures/flow/interfaces-module-and-script/10/output.json

@@ -162,6 +162,7 @@
                 "name": "x"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {
@@ -214,6 +215,7 @@
                 "name": "y"
               },
               "static": true,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {

test/fixtures/flow/interfaces-module-and-script/4/output.json

@@ -112,6 +112,7 @@
                 "name": "foo"
               },
               "static": false,
+              "proto": false,
               "kind": "init",
               "method": false,
               "value": {