Separate Dart backend specific logic from basic Unparser.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index b5a156dea054166c46b30d2ae34c4bedc50a449d..3bcb4a7cbb7b97ddfb73fbfb373c03c3a8076d36 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -155,9 +155,8 @@ class DartBackend extends Backend {
       sortedClassMembers[classElement] = sortElements(members);
     });
 
-    final unparser = new Unparser.withRenamer((Node node) => renames[node]);
     compiler.assembledCode = emitCode(
-        compiler, unparser, imports, sortedTopLevels, sortedClassMembers);
+        compiler, renames, imports, sortedTopLevels, sortedClassMembers);
   }
 
   log(String message) => compiler.log('[DartBackend] $message');