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

…nn-type>.completer.

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!
}
```

This change should be a no-op. Adding this explicit null-assert ensures that any exception is thrown right at the null-assert.

dwds/lib/src/services/batched_expression_evaluator.dart

@@ -156,7 +156,7 @@ class BatchedExpressionEvaluator extends ExpressionEvaluator {
             length: requests.length,
           )
               .then((v) {
-            final result = v.first.value;
+            final result = v.first.value!;
             _logger.fine(
               'Got result out of a batch for ${request.expression}: $result',
             );