Revert "Compute frontend/backend specific constants."

tests/language/const_switch_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.
-
-import 'package:expect/expect.dart';
-
-class C {
-  final field;
-  const C(this.field);
-}
-
-const c1 = const C(0.0);
-const c2 = const C(0);
-const c3 = const C(0.5 + 0.5);
-const c4 = const C(1);
-
-main() {
-  Expect.equals('0.0', test(c1)); /// 01: ok
-  Expect.equals('0', test(c2));   /// 02: ok
-  Expect.equals('1.0', test(c3)); /// 03: ok
-  Expect.equals('1', test(c4));   /// 04: ok
-}
-
-String test(C c) {
-  switch (c) {
-    case const C(0.0): return '0.0';
-    case const C(0): return '0';
-    case const C(1.0): return '1.0';
-    case const C(1): return '1';
-  }
-  return null;
-}