Fix race in debug assertion.

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
===================================================================
--- runtime/vm/thread.cc	(revision 45788)
+++ runtime/vm/thread.cc	(working copy)
@@ -54,6 +54,7 @@
   Thread* thread = Thread::Current();
   ASSERT(thread->isolate() == NULL);
   ASSERT(isolate->mutator_thread() == NULL);
+  thread->isolate_ = isolate;
   isolate->set_mutator_thread(thread);
   // TODO(koda): Migrate thread_state_ and profile_data_ to Thread, to allow
   // helper threads concurrent with mutator.
@@ -67,7 +68,6 @@
   Profiler::BeginExecution(isolate);
   isolate->set_thread_state(thread_state);
   isolate->set_vm_tag(VMTag::kVMTagId);
-  thread->isolate_ = isolate;
 }