Correctly handle const constructors with type parameters.

tests/language/const_constructor_test.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.
+
+class A<T> {
+  final int x;
+  const A.x() : x = 42;
+  A();
+}
+
+main() {
+  Expect.equals(42, (const A<int>.x()).x);
+  Expect.equals(42, (new A<int>.x()).x);
+  const A<int>(); /// 01: compile-time error
+}