Make List.remove return boolean.

tools/dom/templates/html/impl/impl_Element.darttemplate

 // 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 $LIBRARYNAME;
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
 class _ChildrenElementList extends ListBase<Element> {
   // Raw Element.
@@ skipping 161 matching lines @@
   }
 
   void replaceRange(int start, int end, Iterable<Element> iterable) {
     throw new UnimplementedError();
   }
 
   void fillRange(int start, int end, [Element fillValue]) {
     throw new UnimplementedError();
   }
 
-  void remove(Object object) {
+  bool remove(Object object) {
     if (object is Element) {
       Element element = object;
       if (identical(element.parentNode, _element)) {
         _element.$dom_removeChild(element);
+        return true;
       }
     }
+    return false;
   }
 
   void removeWhere(bool test(Element element)) {
     _childElements.removeWhere(test);
   }
 
   void retainWhere(bool test(Element element)) {
     _childElements.retainWhere(test);
   }
 
@@ skipping 140 matching lines @@
   }
 
   Element removeAt(int index) {
     throw new UnsupportedError('');
   }
 
   Element removeLast() {
     throw new UnsupportedError('');
   }
 
-  void remove(Object element) {
+  bool remove(Object element) {
     throw new UnsupportedError('');
   }
 
   void removeWhere(bool test(Element element)) {
     throw new UnsupportedError('');
   }
 
   void retainWhere(bool test(Element element)) {
     throw new UnsupportedError('');
   }
@@ skipping 617 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }