Remove DartString from constants.

pkg/compiler/lib/src/ssa/kernel_impact.dart

 // Copyright (c) 2016, 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 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/names.dart';
 import '../compiler.dart';
 import '../constants/expressions.dart';
@@ skipping 276 matching lines @@
     if (isConst && commonElements.isSymbolConstructor(constructor)) {
       ConstantValue value =
           elementAdapter.getConstantValue(node.arguments.positional.first);
       if (!value.isString) {
         throw new SpannableAssertionFailure(
             CURRENT_ELEMENT_SPANNABLE,
             "Unexpected constant value in const Symbol(...) call: "
             "${value.toStructuredText()}");
       }
       StringConstantValue stringValue = value;
-      impactBuilder
-          .registerConstSymbolName(stringValue.primitiveValue.slowToString());
+      impactBuilder.registerConstSymbolName(stringValue.primitiveValue);
     }
   }
 
   @override
   void visitSuperInitializer(ir.SuperInitializer node) {
     ConstructorEntity target = elementAdapter.getConstructor(node.target);
     _visitArguments(node.arguments);
     impactBuilder.registerStaticUse(new StaticUse.superConstructorInvoke(
         target, elementAdapter.getCallStructure(node.arguments)));
   }
@@ skipping 287 matching lines @@
     ConstructorEntity target = elementAdapter.getConstructor(node.target);
     impactBuilder.registerStaticUse(new StaticUse.superConstructorInvoke(
         target, elementAdapter.getCallStructure(node.arguments)));
   }
 
   // TODO(johnniwinther): Make this throw and visit child nodes explicitly
   // instead to ensure that we don't visit unwanted parts of the ir.
   @override
   void defaultNode(ir.Node node) => node.visitChildren(this);
 }