Remove DartString from constants.

pkg/compiler/lib/src/js_backend/constant_emitter.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.
 
 import '../common.dart';
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
 import '../io/code_output.dart';
@@ skipping 147 matching lines @@
     }
   }
 
   /**
    * Write the contents of the quoted string to a [CodeBuffer] in
    * a form that is valid as JavaScript string literal content.
    * The string is assumed quoted by double quote characters.
    */
   @override
   jsAst.Expression visitString(StringConstantValue constant, [_]) {
-    return js.escapedString(constant.primitiveValue.slowToString(),
-        ascii: true);
+    return js.escapedString(constant.primitiveValue, ascii: true);
   }
 
   @override
   jsAst.Expression visitList(ListConstantValue constant, [_]) {
     List<jsAst.Expression> elements = constant.entries
         .map(constantReferenceGenerator)
         .toList(growable: false);
     jsAst.ArrayInitializer array = new jsAst.ArrayInitializer(elements);
     jsAst.Expression value = makeConstantList(array);
     return maybeAddTypeArguments(constant.type, value);
@@ skipping 116 matching lines @@
             "Unexpected DummyConstantKind ${constant.kind}");
         return null;
     }
   }
 
   @override
   jsAst.Expression visitConstructed(ConstructedConstantValue constant, [_]) {
     ClassElement element = constant.type.element;
     if (backend.isForeign(element) && element.name == 'JS_CONST') {
       StringConstantValue str = constant.fields.values.single;
-      String value = str.primitiveValue.slowToString();
+      String value = str.primitiveValue;
       return new jsAst.LiteralExpression(stripComments(value));
     }
     jsAst.Expression constructor =
         backend.emitter.constructorAccess(constant.type.element);
     List<jsAst.Expression> fields = <jsAst.Expression>[];
     element.forEachInstanceField((_, FieldElement field) {
       fields.add(constantReferenceGenerator(constant.fields[field]));
     }, includeSuperAndInjectedMembers: true);
     if (backend.rtiNeed.classNeedsRtiField(constant.type.element)) {
       fields.add(_reifiedTypeArguments(constant.type));
@@ skipping 34 matching lines @@
           backend.emitter.emitter, argument, unexpected));
     }
     return new jsAst.ArrayInitializer(arguments);
   }
 
   @override
   jsAst.Expression visitDeferred(DeferredConstantValue constant, [_]) {
     return constantReferenceGenerator(constant.referenced);
   }
 }