Reapply "Access to imports in the VM's runtime mirrors. Extend test coverage of the source mirrors."

tests/lib/mirrors/library_imports_metadata.dart

 // Copyright (c) 2014, 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.
 
-// Successful test: output div contains "future!".
+library library_imports_metadata;
+
+@m1
+import 'dart:mirrors' as mirrors;
+
+@m2 @m3
+import 'dart:collection';
 
 import 'dart:async';
 
-const greeting = "Hello, cool people doing nice things!";
-
-// Prints a greeting.
-void main() {
-  print(greeting);
-
-  new Future(() => print("future!"));
-}
+const m1 = const Object();
+const m2 = const Object();
+const m3 = const Object();