Redo "Constant fold charCodeAt via constant system"

sdk/lib/_internal/compiler/implementation/constant_system.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 dart2js;
 
 abstract class Operation {
   String get name;
 }
 
@@ skipping 28 matching lines @@
   BinaryOperation get less;
   BinaryOperation get modulo;
   BinaryOperation get multiply;
   UnaryOperation get negate;
   UnaryOperation get not;
   BinaryOperation get shiftLeft;
   BinaryOperation get shiftRight;
   BinaryOperation get subtract;
   BinaryOperation get truncatingDivide;
 
+  BinaryOperation get codeUnitAt;
+
   const ConstantSystem();
 
   ConstantValue createInt(int i);
   ConstantValue createDouble(double d);
   ConstantValue createString(DartString string);
   ConstantValue createBool(bool value);
   ConstantValue createNull();
   ConstantValue createMap(Compiler compiler,
                           InterfaceType type,
                           List<ConstantValue> keys,
@@ skipping 40 matching lines @@
       case ">>":  return shiftRight;
       case "<":   return less;
       case "<=":  return lessEqual;
       case ">":   return greater;
       case ">=":  return greaterEqual;
       case "==":  return equal;
       default:    return null;
     }
   }
 }