Revert "Constant fold charCodeAt via constant system"

tests/compiler/dart2js/constant_folding_codeUnitAt_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.
-// Test constant folding on numbers.
-
-import 'dart:async';
-import 'package:expect/expect.dart';
-import 'package:async_helper/async_helper.dart';
-import 'compiler_helper.dart';
-
-const String TEST_1 = """
-foo() {
-  var a = 'Hello';
-  var b = 0;
-  return a.codeUnitAt(b);
-}
-""";
-
-// No folding of index type error.
-const String TEST_2 = """
-foo() {
-  var a = 'Hello';
-  var b = 1.5;
-  return a.codeUnitAt(b);
-}
-""";
-
-// No folding of index range error.
-const String TEST_3 = """
-foo() {
-  var a = 'Hello';
-  var b = 55;
-  return a.codeUnitAt(b);
-}
-""";
-
-main() {
-  asyncTest(() => Future.wait([
-    compileAndMatch(TEST_1, 'foo', new RegExp(r'return 72')),
-    compileAndDoNotMatch(TEST_1, 'foo', new RegExp(r'Hello')),
-
-    compileAndMatch(TEST_2, 'foo', new RegExp(r'Hello')),
-    compileAndMatch(TEST_3, 'foo', new RegExp(r'Hello')),
-  ]));
-}