Implement for-in.

frog/leg/resolver.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 TreeElements {
   Map<Node, Element> map;
   TreeElements() : map = new LinkedHashMap<Node, Element>();
   operator []=(Node node, Element element) => map[node] = element;
   operator [](Node node) => map[node];
 }
@@ skipping 471 matching lines @@
 
   visitBreakStatement(BreakStatement node) {
     cancel(node, 'unimplemented');
   }
 
   visitContinueStatement(ContinueStatement node) {
     cancel(node, 'unimplemented');
   }
 
   visitForInStatement(ForInStatement node) {
-    cancel(node, 'unimplemented');
+    visit(node.expression);
+    Scope scope = new BlockScope(context);
+    Node declaration = node.declaredIdentifier;
+    visitIn(declaration, scope);
+    visitIn(node.body, scope);
+    if (declaration is !VariableDefinitions ||
+        !declaration.definitions.nodes.tail.isEmpty()) {
+      // The variable declaration is either not a declaration, or it's
+      // declaring more than one variable.
+      // TODO(lrn): A non-terminating error report would be fine here.
+      print(declaration);
+      cancel(node.declaredIdentifier, "Invalid variable declaration");
+    }
   }
 
   visitLabelledStatement(LabelledStatement node) {
     cancel(node, 'unimplemented');
   }
 
   visitLiteralMap(LiteralMap node) {
     cancel(node, 'unimplemented');
   }
 
@@ skipping 211 matching lines @@
 class TopScope extends Scope {
   Universe universe;
 
   TopScope(Universe this.universe) : super(null, null);
   Element lookup(SourceString name) => universe.find(name);
 
   Element add(Element element) {
     throw "Cannot add an element in the top scope";
   }
 }