Remove unnecessary imports to tree.dart

pkg/compiler/lib/src/inferrer/type_graph_nodes.dart

 // Copyright (c) 2013, 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 compiler.src.inferrer.type_graph_nodes;
 
 import 'dart:collection' show IterableBase;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir show Node;
 import '../dart_types.dart' show DartType, FunctionType, TypeKind;
 import '../elements/elements.dart';
 import '../native/native.dart' as native;
-import '../tree/tree.dart' as ast show DartString, Node, LiteralBool, Send;
+import '../tree/tree.dart' as ast show Node, LiteralBool, Send;
+import '../tree/dartstring.dart' show DartString;
 import '../types/types.dart'
     show
         ContainerTypeMask,
         DictionaryTypeMask,
         MapTypeMask,
         TypeMask,
         ValueTypeMask;
 import '../universe/selector.dart' show Selector;
 import '../util/util.dart' show ImmutableEmptySet, Setlet;
 import '../world.dart' show ClassWorld;
@@ skipping 1174 matching lines @@
   String toString() => 'Type $type';
 
   accept(TypeInformationVisitor visitor) {
     return visitor.visitConcreteTypeInformation(this);
   }
 
   bool hasStableType(TypeGraphInferrerEngine inferrer) => true;
 }
 
 class StringLiteralTypeInformation extends ConcreteTypeInformation {
-  final ast.DartString value;
+  final DartString value;
 
   StringLiteralTypeInformation(value, TypeMask mask)
       : super(new ValueTypeMask(mask, new StringConstantValue(value))),
         this.value = value;
 
   String asString() => value.slowToString();
   String toString() => 'Type $type value ${value.slowToString()}';
 
   accept(TypeInformationVisitor visitor) {
     return visitor.visitStringLiteralTypeInformation(this);
@@ skipping 509 matching lines @@
   } else if (annotation.isVoid) {
     otherType = compiler.typesTask.nullType;
   } else {
     assert(annotation.isInterfaceType);
     otherType = new TypeMask.nonNullSubtype(annotation.element, compiler.world);
   }
   if (isNullable) otherType = otherType.nullable();
   if (type == null) return otherType;
   return type.intersection(otherType, compiler.world);
 }