fix(Testability): fix type error in getAllAngularTestability (dart)

This fixes the following type error that is thrown when calling getAllAngularTestability() while running Dartium in checked mode:

type 'MappedListIterable' is not a subtype of type 'List<PublicTestability>' of 'publicTestabilities'.

Author: goderbauer

modules/angular2/src/core/testability/get_testability.dart

@@ -100,7 +100,7 @@ class GetTestability {
     });
     js.context['getAllAngularTestabilities'] = _jsify(() {
       List<Testability> testabilities = registry.getAllTestabilities();
-      List<PublicTestability> publicTestabilities = testabilities
+      var publicTestabilities = testabilities
           .map((testability) => new PublicTestability(testability));
       return _jsify(publicTestabilities);
     });