Use the constant system to do a subtype type in the compile time constant handler. This is needed b…

tests/language/compile_time_constant_q_test.dart

Index: tests/language/compile_time_constant_q_test.dart
===================================================================
--- tests/language/compile_time_constant_q_test.dart	(revision 0)
+++ tests/language/compile_time_constant_q_test.dart	(revision 0)
@@ -0,0 +1,8 @@
+// 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.
+
+const double x = 14.0;
+main() {
+  print(x);
+}