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

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 7bb53061b26b518f91fc172507b12c0052887b1a..7b5e92e8c93ece35d869341c85db85270d184e06 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -938,6 +938,7 @@ bool Isolate::MakeRunnable() {
   }
   // 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);