* Enforce valid package names on flutter create

Fixes #9564

* refactor

* fix other tests

`all.dart` is no longer needed. Furthermore, it causes tests to
be skipped, or to silently fail to run anything.

Fixes #7941

1. Add matchers for the `ProcessExit` exception class
2. Add ability to control the setup of the `AppContext` we use in
   `testUsingContext()`
3. Clean up the code that figures out the location of `Cache.flutterRoot`
   such that it works with `pub run test`. It previously only worked
   when the tests were invoked with standalone `dart`

`#3` above will also help unblock #7941

This enables us to run more tests on Windows.

Allows for better testing of functions that throw ToolExit to bail out.

- [x] Introduce DependencyChecker which can determine if any dependencies have been modified.
- [x] Move the DartDependencyBuilder into a separate file.
- [x] Add unit tests for DartDependencyBuilder.
- [x] Add unit tets for DependencyChecker

Part of #7014 

* Remove the workaround that pinned args to v0.13.6
This reverts most of the changes in commit 6331b6c8
* throw exception if exit code is not an integer
* rework command infrastructure to throw ToolExit when non-zero exitCode
* convert commands to return Future<Null>
* cleanup remaining commands to use throwToolExit for non-zero exit code
* remove isUnusual exception message
* add type annotations for updated args package

This patch makes `flutter start` work without a clone of the engine git
repository. Making this work pulled a relatively large refactor of how the
commands interact with application packages and devices. Now commands that want
to interact with application packages or devices inherit from a common base
class that holds stores of those objects as members.

In production, the commands download and connect to devices based on the build
configuration stored on the FlutterCommandRunner. In testing, these fields are
used to mock out the real application package and devices.

Also fixes some minor bugs with iOS and Android interactions.

Also update tests to be compatible with the presence of iOS and add tests for list and install.

Refactor all the commands to be Commands from the Args package.  Also use CommandRunner for the top-level command.