Revert "dart2js cps: Add CharCodeAt builtin."

tests/compiler/dart2js/js_backend_cps_ir_codeUnitAt_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.
-
-// Tests for constant folding and lowering String.codeUnitAt.
-
-library basic_tests;
-
-import 'js_backend_cps_ir.dart';
-
-const List<TestEntry> tests = const [
-
-  // Constant folding.
-  const TestEntry(r"""
-main() {
-  print('A'.codeUnitAt(0));
-}""",r"""
-function() {
-  P.print(65);
-}"""),
-
-
-  // Bounds checking.
-  const TestEntry.forMethod('function(foo)',
-r"""
-foo(s) {
-  var sum = 0;
-  for (int i = 0; i < s.length; i++) sum += s.codeUnitAt(i);
-  return sum;
-}
-main() {
-  print(foo('ABC'));
-  print(foo('Hello'));
-}""",r"""
-function(s) {
-  var v0 = s.length, sum = 0, i = 0;
-  for (; i < v0; sum = sum + s.charCodeAt(i), i = i + 1)
-    ;
-  return sum;
-}"""),
-];
-
-
-void main() {
-  runTests(tests);
-}