Redo "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')),
+  ]));
+}