This adds a smoke test for every single API example. It also fixes 17 tests that had bugs in them, or were otherwise broken, and even fixes one actual bug in the framework, and one limitation in the framework.
The bug in the framework is that NetworkImage's _loadAsync method had await response.drain<List<int>>();, but if the response is null, it will throw a cryptic exception saying that Null can't be assigned to List<int>. The fix was just to use await response.drain<void>(); instead.
The limitation is that RelativePositionedTransition takes an Animation<Rect> rect parameter, and if you want to use a RectTween with it, the value emitted there is Rect?, and one of the examples was just casting from Animation<Rect> to Animation<Rect?>, which is invalid, so I modified RelativePositionedTransition to take a Rect? and just use Rect.zero if the rect is null.
This extracts the sample code out from the API doc comments, and places them in separate files on disk, allowing running of the examples locally, testing them, and building of slightly larger examples.
Switch document generation to use the snippets package instead of the snippets code in the Flutter repo. In the process, some bugs in sample code analysis have been fixed, and I've fixed some more errors in the samples.
This will allow the snippets package to be developed separately from the Flutter repo, and reduce the code in the Flutter repo.
The snippets code is deleted in this PR.
I also converted some comments in the snippet templates to be regular comments instead of doc comments, because having a doc comment block before the imports causes the Dart import sorter to lose the comment. They should have been regular comments in the first place.
The snippets package resides in the assets-for-api-docs repo.
The sample analysis has also been converted to be run in parallel, and I've bumped the Dartdoc version to 1.0.2.
The (new, not yet used) code gen for iOS was setting up a std::map from key codes to logical and physical key codes, but it was using uint32_t, which isn't big enough to hold the Flutter key codes.
Also, iOS needs to be able to filter out function keys, so I added a function key set.
