Fix dartium build break.

vm/debugger_api_impl.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "include/dart_debugger_api.h"
 
 #include "vm/dart_api_impl.h"
 #include "vm/dart_api_state.h"
 #include "vm/debugger.h"
 #include "vm/isolate.h"
@@ skipping 115 matching lines @@
     (*bp_resolved_handler)(bpt->id(), url, bpt->LineNumber());
   } else if (event->type == Debugger::kExceptionThrown) {
     if (exc_thrown_handler == NULL) {
       return;
     }
     Dart_Handle exception = Api::NewHandle(isolate, event->exception->raw());
     Dart_StackTrace trace =
         reinterpret_cast<Dart_StackTrace>(isolate->debugger()->StackTrace());
     (*exc_thrown_handler)(exception, trace);
   } else if (event->type == Debugger::kIsolateCreated) {
-    (*isolate_event_handler)(Api::CastIsolate(event->isolate), kCreated);
+    if (isolate_event_handler != NULL) {
+      (*isolate_event_handler)(Api::CastIsolate(event->isolate), kCreated);
+    }
   } else if (event->type == Debugger::kIsolateInterrupted) {
-    (*isolate_event_handler)(Api::CastIsolate(event->isolate), kInterrupted);
+    if (isolate_event_handler != NULL) {
+      (*isolate_event_handler)(Api::CastIsolate(event->isolate), kInterrupted);
+    }
   } else if (event->type == Debugger::kIsolateShutdown) {
-    (*isolate_event_handler)(Api::CastIsolate(event->isolate), kShutdown);
+    if (isolate_event_handler != NULL) {
+      (*isolate_event_handler)(Api::CastIsolate(event->isolate), kShutdown);
+    }
   } else {
     UNIMPLEMENTED();
   }
 }
 
 
 DART_EXPORT void Dart_SetBreakpointResolvedHandler(
                             Dart_BreakpointResolvedHandler handler) {
   bp_resolved_handler = handler;
   Debugger::SetEventHandler(DebuggerEventHandler);
@@ skipping 547 matching lines @@
   if (lib.IsNull()) {
     return Api::NewError("%s: %"Pd" is not a valid library id",
                          CURRENT_FUNC, library_id);
   }
   lib.set_debuggable(is_debuggable);
   return Api::True(isolate);
 }
 
 
 }  // namespace dart