Remove DartString from constants.

pkg/compiler/lib/src/constants/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.
 
 library dart2js.constant_system;
 
 import '../common_elements.dart' show CommonElements;
 import '../elements/operators.dart';
 import '../elements/types.dart';
-import '../tree/dartstring.dart' show DartString;
 import 'values.dart';
 
 abstract class Operation {
   String get name;
 }
 
 abstract class UnaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
   ConstantValue fold(ConstantValue constant);
 }
@@ skipping 33 matching lines @@
   BinaryOperation get subtract;
   BinaryOperation get truncatingDivide;
 
   BinaryOperation get codeUnitAt;
   UnaryOperation get round;
 
   const ConstantSystem();
 
   ConstantValue createInt(int i);
   ConstantValue createDouble(double d);
-  ConstantValue createString(DartString string);
+  ConstantValue createString(String string);
   ConstantValue createBool(bool value);
   ConstantValue createNull();
   ConstantValue createList(InterfaceType type, List<ConstantValue> values);
   ConstantValue createMap(CommonElements commonElements, InterfaceType type,
       List<ConstantValue> keys, List<ConstantValue> values);
   ConstantValue createType(CommonElements commonElements, DartType type);
   ConstantValue createSymbol(CommonElements commonElements, String text);
 
   // We need to special case the subtype check for JavaScript constant
   // system because an int is a double at runtime.
@@ skipping 61 matching lines @@
         return greaterEqual;
       case BinaryOperatorKind.EQ:
         return equal;
       case BinaryOperatorKind.IF_NULL:
         return ifNull;
       default:
         return null;
     }
   }
 }