Redo "Constant fold charCodeAt via constant system"

sdk/lib/_internal/compiler/implementation/js_backend/constant_system_javascript.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 const JAVA_SCRIPT_CONSTANT_SYSTEM = const JavaScriptConstantSystem();
 
 class JavaScriptBitNotOperation extends BitNotOperation {
   const JavaScriptBitNotOperation();
@@ skipping 156 matching lines @@
       const JavaScriptBinaryArithmeticOperation(const MultiplyOperation());
   final negate = const JavaScriptNegateOperation();
   final not = const NotOperation();
   final shiftLeft =
       const JavaScriptBinaryBitOperation(const ShiftLeftOperation());
   final shiftRight = const JavaScriptShiftRightOperation();
   final subtract =
       const JavaScriptBinaryArithmeticOperation(const SubtractOperation());
   final truncatingDivide = const JavaScriptBinaryArithmeticOperation(
       const TruncatingDivideOperation());
+  final codeUnitAt = const CodeUnitAtRuntimeOperation();
 
   const JavaScriptConstantSystem();
 
   /**
    * Returns true if [value] will turn into NaN or infinity
    * at runtime.
    */
   bool integerBecomesNanOrInfinity(int value) {
     double doubleValue = value.toDouble();
     return doubleValue.isNaN || doubleValue.isInfinite;
@@ skipping 147 matching lines @@
     if (onlyStringKeys) {
       result.add(keyList);
     } else {
       // Add the keys individually to avoid generating an unused list constant
       // for the keys.
       result.addAll(keys);
     }
     result.addAll(values);
     return result;
   }
-}
+}