Cleanup api for isolate state checking.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 18012)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -58,8 +58,6 @@
   HANDLESCOPE(__temp_isolate__);
 
 
-const char* CheckIsolateState(Isolate *isolate);
-
 void SetupErrorResult(Dart_Handle* handle);
 
 
@@ -98,6 +96,10 @@
       const ApiState& state,
       Dart_Handle object);
 
+  // Returns an Error handle if isolate is in an inconsistent state.
+  // Returns a Success handle when no error condition exists.
+  static Dart_Handle CheckIsolateState(Isolate *isolate);
+
   // Casts the internal Isolate* type to the external Dart_Isolate type.
   static Dart_Isolate CastIsolate(Isolate* isolate);