Add "contains" method to Collection.

lib/coreimpl/collections.dart

 // 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.
 
 /**
  * The [Collections] class implements static methods useful when
  * writing a class that implements [Collection] and the [iterator]
  * method.
  */
 class Collections {
+  static bool contains(Iterable iterable, var element) {
+    for (final e in iterable) {
+      if (element == e) return true;
+    }
+    return false;
+  }
+
   static void forEach(Iterable iterable, void f(o)) {
     for (final e in iterable) {
       f(e);
     }
   }
 
   static bool some(Iterable iterable, bool f(o)) {
     for (final e in iterable) {
       if (f(e)) return true;
     }
@@ skipping 125 matching lines @@
    * Returns true if the specified collection contains the specified object
    * reference.
    */
   static _containsRef(Collection c, Object ref) {
     for (var e in c) {
       if (e === ref) return true;
     }
     return false;
   }
 }