Add mirrors tests

test/codegen/lib/mirrors/io_html_mutual_exclusion_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.
+
+library test.io_html_mutual_exclusion;
+
+import 'dart:mirrors';
+
+main() {
+  var libraries = currentMirrorSystem().libraries;
+  bool has_io = libraries[Uri.parse('dart:io')] != null;
+  bool has_html = libraries[Uri.parse('dart:html')] != null;
+
+  if (has_io && has_html) {
+    throw "No embedder should have both dart:io and dart:html accessible";
+  }
+}