dart2js: Use an abstract Name class for names in the generated JavaScript ast.

pkg/compiler/lib/src/js_emitter/code_emitter_task.dart

Index: pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
diff --git a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
index 59fe76ae18539aa14655ccfd8bffa0443f91e259..6f3f30e3e6bfcb759d5e72812e03c21e065f9334 100644
--- a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
+++ b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
@@ -216,8 +216,8 @@ class CodeEmitterTask extends CompilerTask {
           backend.retainMetadataOf(cls);
           oldEmitter.classEmitter.visitFields(cls, false,
               (Element member,
-               String name,
-               String accessorName,
+               jsAst.Name name,
+               jsAst.Name accessorName,
                bool needsGetter,
                bool needsSetter,
                bool needsCheckedSetter) {