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

tests/lib/mirrors/library_exports_shown.dart

Index: tests/lib/mirrors/library_exports_shown.dart
diff --git a/tests/lib/mirrors/library_exports_shown.dart b/tests/lib/mirrors/library_exports_shown.dart
new file mode 100644
index 0000000000000000000000000000000000000000..524ab4c1b286f24651daf1b3e82009529c721759
--- /dev/null
+++ b/tests/lib/mirrors/library_exports_shown.dart
@@ -0,0 +1,10 @@
+// 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 library_exports_shown;
+
+export 'library_imports_a.dart' show somethingFromA, somethingFromBoth;
+export 'library_imports_b.dart' show somethingFromB;
+
+var somethingFromShown;