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
* Gallery scaffolding
* Started RenderSliver
* demo and initial hookup
* Cleaned up demo more and scaffolding basic sliver->widget communication structure.
* works
* states and default indicator building works
* start adding docs
* added an alignment setting optimized the sliver relayout mechanism
* tested a default bottom aligned sized indicator
* Added a bunch of tests
* more fixes and more tests
* Finished the tests
* Add docs
* Add more doc diffing wrt material pull to refresh
* Mention nav bar synergy
* add more asserts
* review 1
* Fix mockito 2 / dart 2 / strong typed tests
* review
* Remove the vscode config
* review
