Reland "Removed default Isolate."

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 5b1e77f20e1ea6305f8db960cf3715091f606946..97854f22eb5b74df4b55da4d37ddf4855180647b 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -478,8 +478,6 @@ class Isolate {
 
   static void GlobalTearDown();
 
-  bool IsDefaultIsolate() const { return this == default_isolate_; }
-
   static void SetCrashIfDefaultIsolateInitialized();
   // Ensures that process-wide resources and the default isolate have been
   // allocated. It is only necessary to call this method in rare cases, for
@@ -1134,14 +1132,12 @@ class Isolate {
     DISALLOW_COPY_AND_ASSIGN(EntryStackItem);
   };
 
-  // This mutex protects highest_thread_id_, thread_data_table_ and
-  // default_isolate_.
+  // This mutex protects highest_thread_id_ and thread_data_table_.
   static Mutex process_wide_mutex_;
 
   static Thread::LocalStorageKey per_isolate_thread_data_key_;
   static Thread::LocalStorageKey isolate_key_;
   static Thread::LocalStorageKey thread_id_key_;
-  static Isolate* default_isolate_;
   static ThreadDataTable* thread_data_table_;
 
   // A global counter for all generated Isolates, might overflow.