Add test case for issue 3147.

tests/standalone/package/packages/package1.dart

Index: tests/standalone/package/packages/package1.dart
===================================================================
--- tests/standalone/package/packages/package1.dart	(revision 0)
+++ tests/standalone/package/packages/package1.dart	(revision 0)
@@ -0,0 +1,14 @@
+// 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.
+
+#library("package1");
+
+#import('package2.dart', prefix: 'p1');
+#import('package2.dart', prefix: 'p2');
+#import('package:package2.dart', prefix: 'p3');
+
+main() {
+  Expect.identical(p1.x, p2.x);
+  Expect.identical(p1.x, p3.x);
+}