Remove legacy syntax support for library import

tests/standalone/package/packages/package1.dart

Index: tests/standalone/package/packages/package1.dart
===================================================================
--- tests/standalone/package/packages/package1.dart	(revision 20447)
+++ tests/standalone/package/packages/package1.dart	(working copy)
@@ -2,11 +2,11 @@
 // 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("package1");
+library package1;
 
-#import('package2.dart', prefix: 'p1');
-#import('package2.dart', prefix: 'p2');
-#import('package:package2.dart', prefix: 'p3');
+import 'package2.dart' as p1;
+import 'package2.dart' as p2;
+import 'package:package2.dart' as p3;
 
 main() {
   Expect.identical(p1.x, p2.x);