Get rid of deprecated methods accessing mutator_thread_ instead of current thread

runtime/vm/exceptions.cc

Index: runtime/vm/exceptions.cc
diff --git a/runtime/vm/exceptions.cc b/runtime/vm/exceptions.cc
index d5d4b3938b218fbd20b2c9f5ac59a7e9e88d575c..82bc54433fbb3a0e622ad300057fff4fa7903b7f 100644
--- a/runtime/vm/exceptions.cc
+++ b/runtime/vm/exceptions.cc
@@ -502,9 +502,8 @@ void Exceptions::ReThrow(Thread* thread,
 
 void Exceptions::PropagateError(const Error& error) {
   Thread* thread = Thread::Current();
-  Isolate* isolate = thread->isolate();
   Zone* zone = thread->zone();
-  ASSERT(isolate->top_exit_frame_info() != 0);
+  ASSERT(thread->top_exit_frame_info() != 0);
   if (error.IsUnhandledException()) {
     // If the error object represents an unhandled exception, then
     // rethrow the exception in the normal fashion.