Use type argument for Future of nullable value in test

[srawlins]

This is a weird one, so I'm going to try to carefully explain the situation and my steps. Here is the code in question:

void _testSimpleValue(String testName, Object? value, Object? expected) {
  // ...
  _testFunction('Future<$testName>', Future.value(value), expected);
  // -------------------------------------^ inferred type argument
  // ...
}

void _testFunction(String testName, Object? value, Object? expected) { /* ... */ }

This code works today. It appears that the type of Future.value(...) inferred by CFE is Future<Object?>. A Future<Object?> constructed with the value constructor, and an Object? argument is sound. However, in https://github.com/dart-lang/language/issues/3304, it was decided that CFE infers the wrong type here, and should infer Future<Object>.

If and when CFE will change its behavior remains to be seen, but analyzer will start reporting an issue that an Object? argument was passed to a Future.value constructor for an argument type of Object, and that would be invalid. The situation there is that Future.value's single parameter is optional, so it's type is T?, but at runtime a null value will throw an exception, so we can pretend the parameter is both required and non-nullable when T is non-nullable.

So to prepare for a CFE change, and support analyzer's new warning, we can just put an explicit type argument.

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