dart:async is optional in Flutter but not in the 2.0.0 SDK

[johnpryan]

The 2.0.0 analyzer will catch usages of dart:async that are missing an import, but the Dart SDK provided by Flutter seems to allow it. This forces users to switch back and forth between SDKs when developing for Flutter and web

• Dart VM version: 2.0.0 (Fri Aug 3 10:53:23 2018 +0200) on "macos_x64"
• MacOSX

[lrhn]

In Dart 2.1, the dart:core library started exporting Future and Stream. Since you can write functions returning Future or Stream without importing dart:async (by using async or async* function bodies), we decided that you should be able to write the type of those functions too without having to import dart:async.

It does mean that any code using that feature should have an SDK requirement of 2.1.0.
The Flutter project has been releasing development versions of the Dart SDK, so they already have the feature, while the stable Dart 2.1 release hasn't been made yet.

(Analyzer people, would it make sense to detect uses of Future and Stream without an import when a package has an SDK requirement <2.1.0?)

[stereotype441]

Related: #34978.

[srawlins]

I'll close this as stale.