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

runtime/vm/isolate.cc

 // Copyright (c) 2013, 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.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
 #include "vm/code_observers.h"
@@ skipping 920 matching lines @@
 bool Isolate::MakeRunnable() {
   ASSERT(Isolate::Current() == NULL);
 
   MutexLocker ml(mutex_);
   // Check if we are in a valid state to make the isolate runnable.
   if (is_runnable_ == true) {
     return false;  // Already runnable.
   }
   // Set the isolate as runnable and if we are being spawned schedule
   // isolate on thread pool for execution.
+  ASSERT(object_store()->root_library() != Library::null());
   is_runnable_ = true;
   if (!ServiceIsolate::IsServiceIsolate(this)) {
     message_handler()->set_pause_on_start(FLAG_pause_isolates_on_start);
     message_handler()->set_pause_on_exit(FLAG_pause_isolates_on_exit);
   }
   IsolateSpawnState* state = spawn_state();
   if (state != NULL) {
     ASSERT(this == state->isolate());
     Run();
   }
@@ skipping 1019 matching lines @@
                            serialized_message_, serialized_message_len_);
 }
 
 
 void IsolateSpawnState::Cleanup() {
   SwitchIsolateScope switch_scope(I);
   Dart::ShutdownIsolate();
 }
 
 }  // namespace dart