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

lib/html/templates/html/impl/impl_Node.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.
 
 /**
  * Lazy implementation of the child nodes of an element that does not request
  * the actual child nodes of an element until strictly necessary greatly
  * improving performance for the typical cases where it is not required.
  */
 class _ChildNodeListLazy implements List {
@@ skipping 38 matching lines @@
   }
 
   void operator []=(int index, _NodeImpl value) {
     _this.$dom_replaceChild(value, this[index]);
   }
 
   Iterator<Node> iterator() => _this.$dom_childNodes.iterator();
 
   // TODO(jacobr): We can implement these methods much more efficiently by
   // looking up the nodeList only once instead of once per iteration.
+  bool contains(Node element) => _Collections.contains(this, element);
+
   void forEach(void f(Node element)) => _Collections.forEach(this, f);
 
   Collection map(f(Node element)) => _Collections.map(this, [], f);
 
   Collection<Node> filter(bool f(Node element)) =>
      new _NodeListWrapper(_Collections.filter(this, <Node>[], f));
 
   bool every(bool f(Node element)) => _Collections.every(this, f);
 
   bool some(bool f(Node element)) => _Collections.some(this, f);
@@ skipping 68 matching lines @@
       final _NodeImpl parent = this.parent;
       parent.$dom_replaceChild(otherNode, this);
     } catch (e) {
 
     };
     return this;
   }
 
 $!MEMBERS
 }