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

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

@@ -293,20 +293,23 @@ abstract class WidgetsBinding extends BindingBase with SchedulerBinding, Gesture
       }
     );
 
-    // This service extension is deprecated and will be removed by 7/1/2018.
-    // Use ext.flutter.inspector.show instead.
-    registerBoolServiceExtension(
-        name: 'debugWidgetInspector',
-        getter: () async => WidgetsApp.debugShowWidgetInspectorOverride,
-        setter: (bool value) {
-          if (WidgetsApp.debugShowWidgetInspectorOverride == value)
-            return new Future<Null>.value();
-          WidgetsApp.debugShowWidgetInspectorOverride = value;
-          return _forceRebuild();
-        }
-    );
+    assert(() {
+      // This service extension is deprecated and will be removed by 7/1/2018.
+      // Use ext.flutter.inspector.show instead.
+      registerBoolServiceExtension(
+          name: 'debugWidgetInspector',
+          getter: () async => WidgetsApp.debugShowWidgetInspectorOverride,
+          setter: (bool value) {
+            if (WidgetsApp.debugShowWidgetInspectorOverride == value)
+              return new Future<Null>.value();
+            WidgetsApp.debugShowWidgetInspectorOverride = value;
+            return _forceRebuild();
+          }
+      );
 
-    WidgetInspectorService.instance.initServiceExtensions(registerServiceExtension);
+      WidgetInspectorService.instance.initServiceExtensions(registerServiceExtension);
+      return true;
+    }());
   }
 
   Future<Null> _forceRebuild() {