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

lib/html/src/_Collections.dart

 // Copyright (c) 2011, 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.
 
 /**
  * The [Collections] class implements static methods useful when
  * writing a class that implements [Collection] and the [iterator]
  * method.
  */
 class _Collections {
+  static bool contains(Iterable<Object> iterable, Object element) {
+    for (final e in iterable) {
+      if (e == element) return true;
+    }
+    return false;
+  }
+
   static void forEach(Iterable<Object> iterable, void f(Object o)) {
     for (final e in iterable) {
       f(e);
     }
   }
 
   static List map(Iterable<Object> source,
                   List<Object> destination,
                   f(o)) {
     for (final e in source) {
@@ skipping 22 matching lines @@
     for (final e in source) {
       if (f(e)) destination.add(e);
     }
     return destination;
   }
 
   static bool isEmpty(Iterable<Object> iterable) {
     return !iterable.iterator().hasNext();
   }
 }