Generate contains() method for list mixin only if not already defined.

lib/html/templates/immutable_list_mixin.darttemplate

   // -- start List<$E> mixins.
   // $E is the element type.
 
   // From Iterable<$E>:
 
   Iterator<$E> iterator() {
     // Note: NodeLists are not fixed size. And most probably length shouldn't
     // be cached in both iterator _and_ forEach method. For now caching it
     // for consistency.
     return new _FixedSizeListIterator<$E>(this);
   }
 
   // From Collection<$E>:
 
   void add($E value) {
     throw const UnsupportedOperationException("Cannot add to immutable List.");
   }
 
   void addLast($E value) {
     throw const UnsupportedOperationException("Cannot add to immutable List.");
   }
 
   void addAll(Collection<$E> collection) {
     throw const UnsupportedOperationException("Cannot add to immutable List.");
   }
 
+$if DEFINE_CONTAINS
+  bool contains($E element) => _Collections.contains(this, element);
+$else
+  // contains() defined by IDL.
+$endif
+
   void forEach(void f($E element)) => _Collections.forEach(this, f);
 
   Collection map(f($E element)) => _Collections.map(this, [], f);
 
   Collection<$E> filter(bool f($E element)) =>
      _Collections.filter(this, <$E>[], f);
 
   bool every(bool f($E element)) => _Collections.every(this, f);
 
   bool some(bool f($E element)) => _Collections.some(this, f);
@@ skipping 29 matching lines @@
   }
 
   void insertRange(int start, int rangeLength, [$E initialValue]) {
     throw const UnsupportedOperationException("Cannot insertRange on immutable List.");
   }
 
   List<$E> getRange(int start, int rangeLength) =>
       _Lists.getRange(this, start, rangeLength, <$E>[]);
 
   // -- end List<$E> mixins.