Avoid passing a nullable value to Future<nn-type>.value (#3567)

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!
}
```

- Thanks for your contribution! Please replace this text with a description of what this PR is changing or adding and why, list any relevant issues, and review the contribution guidelines below.

---

<details>
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</details>

build_test/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.2.1-dev
+
+- Avoid passing a nullable value to `Future.value`.
+
 ## 2.2.0
 
 - Forward logs from `testBuilder` to `printOnFailure` by default.

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

build_test/pubspec.yaml

@@ -1,6 +1,6 @@
 name: build_test
 description: Utilities for writing unit tests of Builders.
-version: 2.2.0
+version: 2.2.1-dev
 repository: https://github.com/dart-lang/build/tree/master/build_test
 
 environment: