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

pkg/compiler/lib/src/types/constants.dart

Index: pkg/compiler/lib/src/types/constants.dart
diff --git a/pkg/compiler/lib/src/types/constants.dart b/pkg/compiler/lib/src/types/constants.dart
index ec39d45da075fc4c498fefe151dd7bc8e04ca915..1bcc1bef60fec8b8f385213e7931a604d7942d2e 100644
--- a/pkg/compiler/lib/src/types/constants.dart
+++ b/pkg/compiler/lib/src/types/constants.dart
@@ -54,6 +54,8 @@ class ConstantValueTypeMasks extends ConstantValueVisitor<TypeMask, Compiler> {
         return constant.payload;
       case SyntheticConstantKind.TYPEVARIABLE_REFERENCE:
         return compiler.typesTask.intType;
+      case SyntheticConstantKind.NAME:
+        return compiler.typesTask.stringType;
       default:
         compiler.internalError(compiler.currentElement,
                                "Unexpected DummyConstantKind.");