Avoid ever creating the WidgetInspectorService outside of debug mode. (#21648)

packages/flutter/lib/src/widgets/binding.dart

@@ -293,6 +293,7 @@ abstract class WidgetsBinding extends BindingBase with SchedulerBinding, Gesture
       }
     );
 
+    assert(() {
       // This service extension is deprecated and will be removed by 7/1/2018.
       // Use ext.flutter.inspector.show instead.
       registerBoolServiceExtension(
@@ -307,6 +308,8 @@ abstract class WidgetsBinding extends BindingBase with SchedulerBinding, Gesture
       );
 
       WidgetInspectorService.instance.initServiceExtensions(registerServiceExtension);
+      return true;
+    }());
   }
 
   Future<Null> _forceRebuild() {