Promote Stephen's InterfaceInheritFieldTest to a language test.

frog/tests/frog/src/InterfaceInheritFieldTest.dart

Index: frog/tests/frog/src/InterfaceInheritFieldTest.dart
diff --git a/frog/tests/frog/src/InterfaceInheritFieldTest.dart b/frog/tests/frog/src/InterfaceInheritFieldTest.dart
deleted file mode 100644
index 9aefbf9bff482e2ba9a021c13b18f144693d0efa..0000000000000000000000000000000000000000
--- a/frog/tests/frog/src/InterfaceInheritFieldTest.dart
+++ /dev/null
@@ -1,23 +0,0 @@
-// 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.
-
-// Test that it is legal to override a field with a field in an interface.
-
-interface IA {
-  final int foo;
-}
-
-interface IB extends IA {
-  final int foo;
-}
-
-class B implements IB {
-  int _f = 123;
-  int get foo() => _f;
-}
-
-main() {
-  IB b = new B();
-  print('b.foo = ${b.foo}');
-}