Avoid passing a nullable value to Future<nn-type>.value

This is cleanup work required to start enforcing this with static analysis, as per dart-lang/sdk#53253.

Real quick this issue is that this code is unsafe:

```dart
void f(Completer<int> c, int? i) {
  Future<int>.value(i); // Ouch!
  c.complete(i);        // Ouch!
}
```

build_test/lib/src/written_asset_reader.dart

@@ -51,7 +51,7 @@ class WrittenAssetReader extends MultiPackageAssetReader {
     if (!source.assets.containsKey(id)) {
       throw AssetNotFoundException(id);
     }
-    return Future.value(source.assets[id]);
+    return Future.value(source.assets[id]!);
   }
 
   @override