When setState() calls occur asynchronously, it's possible that the
Futures on which they're waiting don't complete until the widget is
offscreen. To avoid this, we check the widget is mounted before calling
setState().
I'm moving the assets in the assets-for-api-docs repo to a slightly different location to help with organization in that repo, so this PR points the doc URLs to the new location. The old assets won't be removed until this PR makes its way to the API docs website.
No documentation or code changes here, other than changing doc image URLs.
Changes since last roll:
```
ed303c628 Roll dart sdk again. Previous roll required 23ae4fa098 revert. (#4966)
8cd272733 Revert "Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)" (#4965)
9199b40f2 Revert "Support multiple shells in a single process. (#4932)" (#4964)
6baff4c82 Support multiple shells in a single process. (#4932)
31c5bb427 Roll dart to 7764e6962e22afcf4b58c4e3cef3147330f3c884. (#4960)
c8e4c6984 Avoid copying the contents of large platform message responses (#4947)
5ff527295 Update to use new vulkan GrBackendRenderTarget ctor. (#4962)
0c8993a1a Update to use new vulkan GrBackendRenderTarget ctor (part 2) (#4963)
132ebdda8 Revert "Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)" (#4961)
11882ab9e Roll src/third_party/skia/ 9874bf1bc..52e16d984 (135 commits) (#4958)
```
Add consts
This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.
The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.
Applies horizontal safe area insets to the video demo in the Gallery.
This is to support the iPhone X sensor housing notch and other similarly
creative display features when in landscape orientation.
