Compile incrementors in for loops within the initializer's flow

It turns out that incrementors were compiled with the body's flow, which
meant that the incrementor had access to local variables declared in the
body. Now, incrementors no longer have access to such variables.

Fixes #2825.

src/compiler.ts

@@ -2600,6 +2600,7 @@ export class Compiler extends DiagnosticEmitter {
     if (possiblyLoops) {
       let incrementor = statement.incrementor;
       if (incrementor) {
+        this.currentFlow = flow;
         bodyStmts.push(
           this.compileExpression(incrementor, Type.void, Constraints.ConvImplicit | Constraints.WillDrop)
         );

tests/compiler/issues/2826.json

@@ -0,0 +1,5 @@
+{
+  "stderr": [
+    "TS2304: Cannot find name 'j'."
+  ]
+}

tests/compiler/issues/2826.ts

@@ -0,0 +1,3 @@
+for (let i = 0; i < 2; j++) {
+  let j = 0;
+}