This PR adds TargetPlatform.macOS to the TargetPlatform enum. This allows us to begin implementation of some adaptive UI based on which target platform is desired.
I haven't updated the tests here, that will come in a follow-up PR.
I updated the build files for manual_tests, as well as adding in a macos and web directory to allow the manual tests to be run on the web or on desktop.
The main change here are the parts that I added to the files in manual_tests/lib/... (the addition of kIsWeb to "if (!kIsWeb && Platform.isMacOS) {") The rest is just an update of the auto-generated code from flutter create.
* Make CupertinoApp and MaterialApp both use WidgetsApp for Navigator
* Make CupertinoApp and MaterialApp const constructors
* Make WidgetsApp routes aware
* Update tests
* Roll engine to b6df7a637498ca9beda1fa9cd7210e3202ea599f.
Changes since last roll:
```
b6df7a637 Roll dart to 290c576264faa096a0b3206c71b2435309d9f904. (#4771)
a6764dbd5 Add sources for Fuchsia target. (#4763)
2d5900615 [fuchsia] Remove unused header file. (#4769)
9717063b7 Revert "Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)" (#4768)
9a9814312 Roll dart to c080951d45e79cd25df98036c4be835b284a269c. (#4767)
e74e8b35c [async] Update includes of async headers to new path (#4760)
e2c4b2760 Use Dart 2 camel case constants in the engine Dart libraries (#4766)
9c1e48434 Updates for Fuchsia roll. (#4765)
14c940e27 Switch from fxl::Mutex to std::mutex (#4764)
debf82c0b Roll Garnet (#4759)
5bffdefbb Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)
```
