Add asserts that isolates have a root library set up before they become runnable.

tests/lib/mirrors/spawn_function_root_library_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 'dart:mirrors';
+import 'dart:isolate';
+import 'package:expect/expect.dart';
+
+child(SendPort port) {
+  LibraryMirror root = currentMirrorSystem().isolate.rootLibrary;
+  Expect.isNotNull(root);
+  port.send(root.uri.toString());
+}
+
+main() {
+  var port;
+  port = new RawReceivePort((String childRootUri) {
+    LibraryMirror root = currentMirrorSystem().isolate.rootLibrary;
+    Expect.isNotNull(root);
+    Expect.equals(root.uri.toString(), childRootUri);
+    port.close();
+  });
+
+  Isolate.spawn(child, port.sendPort);
+}