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, code like this should no longer compile:

for (let i = 0; i < 2; j++) { let j = 0; }

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)
         );