Handle modifiers (abstract) on class nodes.

tests/compiler/dart2js/unparser_test.dart

Index: tests/compiler/dart2js/unparser_test.dart
diff --git a/tests/compiler/dart2js/unparser_test.dart b/tests/compiler/dart2js/unparser_test.dart
index ddf2b6b9f38aa27c85883308801f623a5dec7e45..af53532804c88be8741d9ef1ce9423609b5a2a17 100644
--- a/tests/compiler/dart2js/unparser_test.dart
+++ b/tests/compiler/dart2js/unparser_test.dart
@@ -243,6 +243,11 @@ testRedirectingFactoryConstructors() {
   testUnparseMember("external const factory Foo() = prefix.Bar<T>.baz;");
 }
 
+testClassDeclarations() {
+  testUnparseTopLevelWithMetadata('class Foo{}');
+  testUnparseTopLevelWithMetadata('abstract class Foo{}');
+}
+
 main() {
   testSignedConstants();
   testGenericTypes();
@@ -261,4 +266,5 @@ main() {
   testPartOf();
   testCombinators();
   testRedirectingFactoryConstructors();
+  testClassDeclarations();
 }