Promote Stephen's InterfaceInheritFieldTest to a language test.

tests/language/src/InterfaceInheritFieldTest.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.
 
 // 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}');
 }