Remove unnecessary imports to tree.dart

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

Index: pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
index dedb4e2ecbdfab549799619f05147a4d5c1f581a..1a6842f9e9b00a671a65b58124b0e0214a71676f 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_nodes.dart
@@ -14,7 +14,8 @@ 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,
@@ -1209,7 +1210,7 @@ class ConcreteTypeInformation extends TypeInformation {
 }
 
 class StringLiteralTypeInformation extends ConcreteTypeInformation {
-  final ast.DartString value;
+  final DartString value;
 
   StringLiteralTypeInformation(value, TypeMask mask)
       : super(new ValueTypeMask(mask, new StringConstantValue(value))),