Add the cast equivalent of assertSubtype and assertSubtypeOfRuntimeType.

tests/language/issue10747_test.dart

Index: tests/language/issue10747_test.dart
===================================================================
--- tests/language/issue10747_test.dart	(revision 0)
+++ tests/language/issue10747_test.dart	(revision 0)
@@ -0,0 +1,28 @@
+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+
+class B<T> {
+}
+
+class A<T> {
+  var field;
+  A(this.field);
+  asTypeVariable() => field as T;
+  asBOfT() => field as B<T>;
+}
+
+main() {
+  Expect.equals(42, new A<int>(42).asTypeVariable());
+  Expect.throws(
+      () => new A<String>(42).asTypeVariable(),
+      (e) => e is CastError);
+
+  var b = new B<int>();
+  Expect.equals(b, new A<int>(b).asBOfT());
+  Expect.throws(
+      () => new A<String>(b).asBOfT(),
+      (e) => e is CastError);
+}