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

tests/lib/mirrors/library_imports_prefixed.dart

Index: tests/lib/mirrors/library_imports_prefixed.dart
diff --git a/tests/lib/mirrors/library_imports_prefixed.dart b/tests/lib/mirrors/library_imports_prefixed.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ad50f424766e6004437ffed6b4b4dd7ac25425b9
--- /dev/null
+++ b/tests/lib/mirrors/library_imports_prefixed.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_imports_prefixed;
+
+import 'library_imports_a.dart' as prefixa;
+import 'library_imports_b.dart' as prefixb;
+
+var somethingFromPrefixed;