Refactor isolate interrupts to use OOB messages instead of interrupt bits.

runtime/vm/debugger.h

Index: runtime/vm/debugger.h
diff --git a/runtime/vm/debugger.h b/runtime/vm/debugger.h
index 685e5b5714a6324b949a728360eb262805ca95b8..cc26fdcff34d7b2ff259adcea9cd1000d9ac23a4 100644
--- a/runtime/vm/debugger.h
+++ b/runtime/vm/debugger.h
@@ -556,14 +556,14 @@ class Debugger {
   RawObject* GetStaticField(const Class& cls,
                             const String& field_name);
 
-  void SignalBpReached();
-  void DebuggerStepCallback();
+  RawError* SignalBpReached();
+  RawError* DebuggerStepCallback();
+  RawError* SignalIsolateInterrupted();
 
   void BreakHere(const String& msg);
 
   void SignalExceptionThrown(const Instance& exc);
   void SignalIsolateEvent(DebuggerEvent::EventType type);
-  static void SignalIsolateInterrupted();
 
   uword GetPatchedStubAddress(uword breakpoint_address);