All temporary directory start with `flutter_` and have their random component separated from the name by a period, as in `flutter_test_bundle.YFYQMY`.
I've tried to find some of the places where we didn't cleanly delete temporary directories, too. This greatly reduces, though it does not entirely eliminate, the directories we leave behind when running tests, especially `flutter_tools` tests.
While I was at it I standardized on `tempDir` as the variable name for temporary directories, since it was the most common, removing occurrences of `temp` and `tmp`, among others.
Also I factored out some common code that used to catch exceptions that happen on Windows, and made more places use that pattern.
* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test
* Make the current command injected into the AppContext, allowing
other classes to inject the current command.
* Introduce `AssetBundleFactory`, an injected factory class for
spawning instances of `AssetBundle`. This allows other run contexts
to use custom asset bundling logic.
* Clean up RunCommand by removing a 'packages' argument that duplicated
a global argument by the same name (and for the same purpose).
Duplicate arguments are confusing and error-prone.
