Deal with unhandled exceptions the same way in all Dart api functions.

runtime/bin/eventhandler.cc

Index: runtime/bin/eventhandler.cc
===================================================================
--- runtime/bin/eventhandler.cc	(revision 1469)
+++ runtime/bin/eventhandler.cc	(working copy)
@@ -18,7 +18,7 @@
   intptr_t value = 0;
   Dart_Handle result = Dart_GetNativeInstanceField(
       handle, kNativeEventHandlerFieldIndex, &value);
-  ASSERT(Dart_IsValid(result));
+  ASSERT(!Dart_IsError(result));
   EventHandler* event_handler = reinterpret_cast<EventHandler*>(value);
   ASSERT(event_handler != NULL);
   return event_handler;