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

tests/lib/mirrors/library_imports_prefixed_show_hide.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.
 
-class A {
-  static var a;
-  var b;
-}
+library library_imports_prefixed_show_hide;
 
-class B extends A {
+import 'library_imports_a.dart' as prefixa show somethingFromA;
+import 'library_imports_b.dart' as prefixb hide somethingFromB;
 
-}
-
-class C extends B {
-  var a; /// 01: static type warning
-  static var b; /// 02: compile-time error
-}
-
-void main() {
-  new C();
-}
+var somethingFromPrefixed;