Flow analysis: promote to "types of interest" on assignment.

This allows so-called "ensure guarded" promotions, e.g.
- `if (x is! int) x = 0;`
- `if (x == null) x = 0;`
- `x ??= 0;`

The latter two are particularly important to prepare for NNBD.

Change-Id: Ib53ca916e6d485945326b19e86f8cfb5b3ee2160
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/123280
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analyzer/lib/src/dart/resolver/flow_analysis_visitor.dart

@@ -77,7 +77,7 @@ class FlowAnalysisHelper {
     if (flow == null) return null;
 
     if (node.operator.type == TokenType.QUESTION_QUESTION_EQ) {
-      flow.ifNullExpression_rightBegin();
+      flow.ifNullExpression_rightBegin(node.leftHandSide);
     }
 
     var left = node.leftHandSide;

pkg/analyzer/lib/src/generated/resolver.dart

@@ -3130,7 +3130,7 @@ class ResolverVisitor extends ScopedVisitor {
       }
 
       if (operator == TokenType.QUESTION_QUESTION) {
-        flow?.ifNullExpression_rightBegin();
+        flow?.ifNullExpression_rightBegin(node.leftOperand);
         right.accept(this);
         flow?.ifNullExpression_end();
       } else {