Add language tests for Issue 18628.

tests/language/issue18628_2_test.dart

+// Copyright (c) 2014, 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.
+
+// This test checks for a regression found in Dart Editor: the
+// analyzer was treating [Type] as more specific than any type
+// variable (generic parameter).
+//
+// https://code.google.com/p/dart/issues/detail?id=18628
+
+class X<T extends Type> {}
+
+// This line is supposed to cause the warning; the other lines are
+// marked because they don't make sense when [Y] is not defined.
+class Y<U> extends X<U> {} /// 01: static type warning
+
+main() {
+  X<Type> x = new X<Type>(); /// 01: static type warning
+  Y<Type> y = new Y<Type>(); /// 01: static type warning
+}