List.add is void (returns null). Fix issue 1213.

runtime/lib/growable_array.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.
 
 class GrowableObjectArray<T> implements List<T> {
   ObjectArray<T> backingArray;
 
   factory GrowableObjectArray._uninstantiable() {
     throw const UnsupportedOperationException(
         "GrowableObjectArray can only be allocated by the VM");
@@ skipping 126 matching lines @@
 
   void grow(int capacity) {
     ObjectArray<T> newArray = new ObjectArray<T>(capacity);
     int length = backingArray.length;
     for (int i = 0; i < length; i++) {
       newArray[i] = backingArray[i];
     }
     backingArray = newArray;
   }
 
-  int add(T value) {
+  void add(T value) {
     if (_length == backingArray.length) {
       grow(_length * 2);
     }
     backingArray[_length] = value;
-    return ++_length;
+    ++_length;
   }
 
   void addLast(T element) {
     add(element);
   }
 
   void addAll(Collection<T> collection) {
     for (T elem in collection) {
       add(elem);
     }
@@ skipping 87 matching lines @@
     if (!hasNext()) {
       throw const NoMoreElementsException();
     }
     return _array[_pos++];
   }
 
   final GrowableObjectArray<T> _array;
   int _pos;
 }