Revert "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 55e76fadcf8719007af1ddd26dbdaee5068818bf..07f0abad47e1bd4005746a6a55f85522b184e4a5 100644
--- a/lib/coreimpl/collections.dart
+++ b/lib/coreimpl/collections.dart
@@ -8,13 +8,6 @@
  * 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);