Handle modifiers (abstract) on class nodes.

tests/compiler/dart2js/unparser_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #import('dart:uri');
 #import('parser_helper.dart');
 #import('mock_compiler.dart');
 #import("../../../lib/compiler/implementation/tree/tree.dart");
 
 testUnparse(String statement) {
@@ skipping 225 matching lines @@
   testUnparseMember("external const factory Foo() = Bar;");
   testUnparseMember("external const factory Foo() = Bar.baz;");
   testUnparseMember("external const factory Foo() = Bar<T>;");
   testUnparseMember("external const factory Foo() = Bar<T>.baz;");
   testUnparseMember("external const factory Foo() = prefix.Bar;");
   testUnparseMember("external const factory Foo() = prefix.Bar.baz;");
   testUnparseMember("external const factory Foo() = prefix.Bar<T>;");
   testUnparseMember("external const factory Foo() = prefix.Bar<T>.baz;");
 }
 
+testClassDeclarations() {
+  testUnparseTopLevelWithMetadata('class Foo{}');
+  testUnparseTopLevelWithMetadata('abstract class Foo{}');
+}
+
 main() {
   testSignedConstants();
   testGenericTypes();
   testForLoop();
   testEmptyList();
   testClosure();
   testIndexedOperatorDecl();
   testNativeMethods();
   testPrefixIncrements();
   testConstModifier();
   testSimpleObjectInstantiation();
   testLibraryName();
   testImport();
   testExport();
   testPart();
   testPartOf();
   testCombinators();
   testRedirectingFactoryConstructors();
+  testClassDeclarations();
 }