Move dart2jslib parts into separate libraries.

tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart

Index: tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
diff --git a/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart b/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
index df70bafe58f43c4545adce142df1d491fb05a634..7775b668bee124e0a12715acfac0b49bca60463b 100644
--- a/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
+++ b/tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart
@@ -9,13 +9,13 @@ library sexpr_unstringifier;
 
 import 'package:compiler/src/constants/expressions.dart';
 import 'package:compiler/src/constants/values.dart';
-import 'package:compiler/src/dart2jslib.dart' as dart2js
-    show MessageKind;
 import 'package:compiler/src/dart_types.dart' as dart_types
     show DartType;
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/modelx.dart'
     show ErroneousElementX, TypeVariableElementX;
+import 'package:compiler/src/messages.dart'
+    show MessageKind;
 import 'package:compiler/src/tree/tree.dart' show LiteralDartString;
 import 'package:compiler/src/universe/universe.dart'
     show Selector, SelectorKind, CallStructure;
@@ -48,7 +48,7 @@ class DummyLocal extends DummyEntity implements Local {
 class DummyElement extends ErroneousElementX
     implements TypeVariableElement, FieldElement {
   DummyElement(String name)
-      : super(dart2js.MessageKind.GENERIC, {}, name, null);
+      : super(MessageKind.GENERIC, {}, name, null);
 
   final dart_types.DartType bound = null;
   final TypeDeclarationElement typeDeclaration = null;