Implement for-in.

frog/leg/typechecker.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name() => "Type checker";
 
   void check(Node tree, TreeElements elements) {
     measure(() {
@@ skipping 569 matching lines @@
 
   visitBreakStatement(BreakStatement node) {
     compiler.unimplemented('visitBreakStatement', node: node);
   }
 
   visitContinueStatement(ContinueStatement node) {
     compiler.unimplemented('visitContinueStatement', node: node);
   }
 
   visitForInStatement(ForInStatement node) {
-    compiler.unimplemented('visitForInStatement', node: node);
+    node.visitChildren(this);
   }
 
   visitLabelledStatement(LabelledStatement node) {
     compiler.unimplemented('visitLabelledStatement', node: node);
   }
 
   visitLiteralMap(LiteralMap node) {
     compiler.unimplemented('visitLiteralMap', node: node);
   }
 
@@ skipping 18 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     compiler.unimplemented('visitCatchBlock', node: node);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('visitTypedef', node: node);
   }
 }