discarded_futures lint false positive

[ekuleshov]

Describe the issue
With discarded_futures lint enabled the false positive is triggered when method returning a Future is called from a non-async method and returned future is passed to another method or Flutter widget. In Flutter MultiFrameImageStreamCompleter and FutureBuilder both are taking Future as a parameter.

Similarly, a future could be assigned to a variable and used later in a method, yet still discarded_futures lint is raised.

To Reproduce
Here is a code snippet that is triggering issue. Note that FutureBuilder is usually within Flutter's build() method, which can't be async.

FutureBuilder<Widget?>(
  future: _buildImage(context, ...), // ignore: discarded_futures
  builder: (_, snapshot) => snapshot.hasData ? snapshot.data! : const Box(),
)

Where the _buildImage method is declared like this:

Future<Widget?> _buildImage(context, ...) async {
  ...get image data asynchronously
  return Image(...);
}

**Expected behavior**
This lint should not be raised when future is assigned to a a used local variable or passed down as a method or constructor parameter.

**Additional context**
N/A