Fix the broken build (#5076)

Author: kenzieschmoll

packages/devtools_app/test/shared/service_extension_widgets_test.dart

@@ -27,7 +27,7 @@ Future<void> main() async {
   when(mockServiceManager.appState).thenReturn(
     AppState(mockServiceManager.isolateManager.selectedIsolate),
   );
-  when(mockServiceManager.runDeviceBusyTask(Future<void>.value()))
+  when(mockServiceManager.runDeviceBusyTask(any))
       .thenAnswer((_) => Future<void>.value());
   when(mockServiceManager.isMainIsolatePaused).thenReturn(false);
   setGlobal(ServiceConnectionManager, mockServiceManager);

packages/devtools_app/test/test_infra/goldens/inspector_service_details_tree.txt

@@ -8,7 +8,7 @@ MaterialApp
      └─Focus
        │ state: _FocusState#00000
        │
-       └─_FocusMarker
+       └─_FocusInheritedScope
          └─Semantics
            │ container: false
            │ properties: SemanticsProperties

packages/devtools_test/lib/src/wrappers.dart

@@ -134,7 +134,12 @@ void testWidgetsWithContext(
     } finally {
       // restore previous global values
       for (Type type in oldValues.keys) {
-        setGlobal(type, oldValues[type]);
+        final oldGlobal = oldValues[type];
+        if (oldGlobal != null) {
+          setGlobal(type, oldGlobal);
+        } else {
+          globals.remove(type);
+        }
       }
     }
   });