Allow Observatory debugger to switch isolates.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index c0cfac7d317c8669a81fd1f181b6f96cc49484d1..815f39d03a099fff2f099e914b241d45ab815797 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -169,6 +169,8 @@ class Isolate : public BaseIsolate {
   }
 
   const char* name() const { return name_; }
+  const char* debugger_name() const { return debugger_name_; }
+  void set_debugger_name(const char* name);
   class Log* Log() const;
 
   int64_t start_time() const { return start_time_; }
@@ -672,6 +674,7 @@ class Isolate : public BaseIsolate {
   MegamorphicCacheTable megamorphic_cache_table_;
   Dart_MessageNotifyCallback message_notify_callback_;
   char* name_;
+  char* debugger_name_;
   int64_t start_time_;
   Dart_Port main_port_;
   Dart_Port origin_id_;  // Isolates created by spawnFunc have some origin id.
@@ -783,7 +786,8 @@ class Isolate : public BaseIsolate {
   static void AddIsolateTolist(Isolate* isolate);
   static void RemoveIsolateFromList(Isolate* isolate);
   static void CheckForDuplicateThreadState(InterruptableThreadState* state);
-  static Monitor* isolates_list_monitor_;
+
+  static Monitor* isolates_list_monitor_;  // Protects isolates_list_head_
   static Isolate* isolates_list_head_;
 
 #define REUSABLE_FRIEND_DECLARATION(name)                                      \