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

tests/lib/mirrors/library_imports_bad_metadata_test.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.
-
-library test.library_imports_bad_metadata;
-
-@undefined  /// 01: compile-time error
-import 'dart:mirrors';
-import 'package:expect/expect.dart';
-
-main() {
-  LibraryMirror thisLibrary =
-      currentMirrorSystem().findLibrary(#test.library_imports_bad_metadata);
-
-  thisLibrary.libraryDependencies.forEach((dep) {
-    Expect.listEquals([], dep.metadata);
-  });
-}