Cache current thread in a reserved register and use it in LoadIsolate

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index 9ee689f01d678b031a17a7e634ae72dd28898dea..a7b2486befe0c70d88424377f35a3c8bed18f9f2 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -167,11 +167,15 @@ class Isolate : public BaseIsolate {
 
   // A thread that operates on this isolate and may execute Dart code.
   // No other threads operating on this isolate may execute Dart code.
-  // TODO(koda): Remove after caching current thread in generated code.
+  // TODO(koda): Remove after pivoting to thread in NativeArguments.
   Thread* mutator_thread() {
     DEBUG_ASSERT(mutator_thread_ == NULL || IsIsolateOf(mutator_thread_));
     return mutator_thread_;
   }
+  // TODO(koda): Remove after pivoting to thread in JumpToExceptionHandler.
+  static intptr_t mutator_thread_offset() {
+    return OFFSET_OF(Isolate, mutator_thread_);
+  }
 #if defined(DEBUG)
   bool IsIsolateOf(Thread* thread);
 #endif  // DEBUG