Compile incrementors within the initializer's flow in for loops

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 AssemblyScript#2825.

Author: CountBleck

Diff for: 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)
         );

Diff for: tests/compiler/issues/2826.json

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

Diff for: tests/compiler/issues/2826.ts

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