Remove legacy syntax support for library import

tests/standalone/package/packages/lib2/lib2.dart

Index: tests/standalone/package/packages/lib2/lib2.dart
===================================================================
--- tests/standalone/package/packages/lib2/lib2.dart	(revision 20447)
+++ tests/standalone/package/packages/lib2/lib2.dart	(working copy)
@@ -2,10 +2,10 @@
 // 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.
 
-#library('lib2');
+library lib2;
 
-#import('package:shared.dart');
-#import('package:lib3/sub/lib3.dart');
+import 'package:shared.dart';
+import 'package:lib3/sub/lib3.dart';
 
 void lib2() {
   output += '|lib2';