Make List.remove return boolean.

tools/dom/templates/immutable_list_mixin.darttemplate

   // -- start List<$E> mixins.
   // $E is the element type.
 
   // From Iterable<$E>:
 
   Iterator<$E> get iterator {
     // Note: NodeLists are not fixed size. And most probably length shouldn't
     // be cached in both iterator _and_ forEach method. For now caching it
     // for consistency.
     return new FixedSizeListIterator<$E>(this);
@@ skipping 141 matching lines @@
   }
 
   $E removeAt(int pos) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   $E removeLast() {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
-  void remove(Object object) {
+  bool remove(Object object) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   void removeWhere(bool test($E element)) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
 
   void retainWhere(bool test($E element)) {
     throw new UnsupportedError("Cannot remove from immutable List.");
   }
@@ skipping 30 matching lines @@
     IterableMixinWorkaround.asMapList(this);
 
   String toString() {
     StringBuffer buffer = new StringBuffer('[');
     buffer.writeAll(this, ', ');
     buffer.write(']');
     return buffer.toString();
   }
 
   // -- end List<$E> mixins.