Add one more test for configurable imports.

tests/language/config_import_corelib_test.dart

+// Copyright (c) 2015, 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.
+
+import 'package:expect/expect.dart';
+
+import 'config_import_corelib_general.dart'
+  if (dart.library.io) 'config_import_corelib_io.dart'
+  if (dart.library.http) 'config_import_corelib_http.dart'
+  as lib;
+
+class SubClassy extends lib.Classy {
+  String get superName => super.name;
+}
+
+main() {
+  var io = const bool.fromEnvironment("dart.library.io");
+  var http = const bool.fromEnvironment("dart.library.http");
+
+  var cy = new SubClassy();
+
+  if (io) {
+    Expect.isTrue(lib.general());
+    Expect.equals("io", lib.name);
+    Expect.equals("classy io", cy.name);
+
+    Expect.isTrue(lib.ioSpecific());
+    Expect.equals("classy io", cy.ioSpecific());
+
+    Expect.throws(() { lib.httpSpecific(); });
+    Expect.throws(() { cy.httpSpecific(); });
+  } else if (http) {
+    Expect.isTrue(lib.general());
+    Expect.equals("http", lib.name);
+    Expect.equals("classy http", cy.name);
+
+    Expect.throws(() { lib.ioSpecific(); });
+    Expect.throws(() { cy.ioSpecific(); });
+
+    Expect.isTrue(lib.httpSpecific());
+    Expect.equals("classy http", cy.httpSpecific());
+  } else {
+    Expect.isTrue(lib.general());
+    Expect.equals("general", lib.name);
+    Expect.equals("classy general", cy.name);
+
+    Expect.throws(() { lib.ioSpecific(); });
+    Expect.throws(() { cy.ioSpecific(); });
+
+    Expect.throws(() { lib.httpSpecific(); });
+    Expect.throws(() { cy.httpSpecific(); });
+  }
+}