Add "contains" method to Collection.

lib/coreimpl/collections.dart

Index: lib/coreimpl/collections.dart
diff --git a/lib/coreimpl/collections.dart b/lib/coreimpl/collections.dart
index 07f0abad47e1bd4005746a6a55f85522b184e4a5..55e76fadcf8719007af1ddd26dbdaee5068818bf 100644
--- a/lib/coreimpl/collections.dart
+++ b/lib/coreimpl/collections.dart
@@ -8,6 +8,13 @@
  * 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);