Null-assert the value given to a Completer expecting a non-null value (#136776)

The code with out the null assertion is legal as per the type signature, but will throw a runtime exception if the nullable value is null. To make this exception more explicit, the value must be null-checked before completing the completer with the value.

The analyzer will soon enforce such checks. See https://github.com/dart-lang/sdk/issues/53253.

Fixes https://github.com/flutter/flutter/issues/136775

packages/flutter_tools/lib/src/vmservice.dart

@@ -964,7 +964,7 @@ class FlutterVmService {
       if (event.kind == vm_service.EventKind.kServiceExtensionAdded
           && event.extensionRPC == extensionName) {
         isolateEvents.cancel();
-        extensionAdded.complete(event.isolate);
+        extensionAdded.complete(event.isolate!);
       }
     });