Fix all for-in warnings in dart2js

pkg/compiler/lib/src/tree/nodes.dart

 // Copyright (c) 2012, 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.
 
 part of tree;
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
@@ skipping 554 matching lines @@
 
   bool get isConst {
     return newToken == null || identical(newToken.stringValue, 'const');
   }
 
   Token getBeginToken() => newToken != null ? newToken : send.getBeginToken();
 
   Token getEndToken() => send.getEndToken();
 }
 
-class NodeList extends Node {
+class NodeList extends Node with IterableMixin<Node> {
   final Link<Node> nodes;
   final Token beginToken;
   final Token endToken;
   final String delimiter;
   bool get isEmpty => nodes.isEmpty;
 
   NodeList([this.beginToken, this.nodes, this.endToken, this.delimiter]);
 
   Iterator<Node> get iterator => nodes.iterator;
 
   NodeList.singleton(Node node) : this(null, const Link<Node>().prepend(node));
   NodeList.empty() : this(null, const Link<Node>());
 
   NodeList asNodeList() => this;
 
+  // Override [IterableMixin.toString] with same code as [Node.toString].
+  toString() => unparse(this);
+
+  get length {
+    throw new UnsupportedError('use slowLength() instead of get:length');
+  }
+
   int slowLength() {
     int result = 0;
     for (Link<Node> cursor = nodes; !cursor.isEmpty; cursor = cursor.tail) {
       result++;
     }
     return result;
   }
 
   accept(Visitor visitor) => visitor.visitNodeList(this);
 
@@ skipping 1880 matching lines @@
 
   // VariableDefinitions.
   get metadata => null;
   get type => null;
 
   // Typedef.
   get typeParameters => null;
   get formals => null;
   get typedefKeyword => null;
 }