Disallow const getters.

tests/language/const_getter_test.dart

 // Copyright (c) 2015, 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 'dart:math' as Math;
+// Check that const getters are not allowed.
+
 import 'package:expect/expect.dart';
 
-main() {
-  Expect.equals(4, new C().m());
+class C {
+  const C();
+
+  const /// 01: compile-time error
+  get x => 1;
 }
 
-class C {
-  max(a) => a;
+const /// 02: compile-time error
+get y => 2;
 
-  m() {
-    return max(Math.max(2, 4));
-  }
-}
+main() {
+  Expect.equals(1, const C().x);
+  Expect.equals(2, y);
+}