* Revert "Change github readme links to be served by github (#13929)"

This reverts commit fd516d95.

* Revert " Rename SemanticsFlags to SemanticsFlag (#13994)"

This reverts commit 24e3f705.

* Revert "Make artifact downloading more robust for flaky networks (#13984)"

This reverts commit 4d37e03e.

* First version

* Add a clarifying comment

* Add verify to ZipDecoder.decodeBytes call

* Review comments

* Need OS Error for missing files

* FileSystemException, not OSError.

Add a flutter doctor --android-licenses command that locates and runs the Android SDK license manager (#9892)

See https://github.com/flutter/flutter/issues/8438

The old `whichAll` implementation was not considering different extensions for executables on Windows. By defering to OS-built-in tools we avoid implementing it.

Fixes #8882.

1. Add `PortScanner` abstraction so that we don't do actual port scanning
   in tests.
2. Don't change the real `cwd` of the isolate during tests, as it affects
   all tests, not just the current running test.

Fixes #8761

Fixes #8618

This adds support for a `--bug-report` flag, which is a recording
that:
  - includes the arguments that were passed to the command runner
  - is zipped up for easy attachment in Guthub issues

...and add --diagnostic-port.

...and document port 0.

Required for https://github.com/flutter/flutter/issues/138

This ensures that accidental usages of dart:io's
file API don't creep in over time.

This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.

This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.

With this change, they're run via instance methods on an object
obtained through the context. This will allow us to substitute
that object in tests with replay/record versions to allow us to
mock out the os-layer in tests.

* implement flutter packages get/upgrade command
* alias flutter pub
* refactor findProjectRoot for use by multiple commands

fixes https://github.com/flutter/flutter/issues/4994

* don't resolve symlinks as part of which

* update method dartdoc

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

* better messaging about windows support

* fix lints

* move the engine download code to dart

* fix populating the bin/cache/pkg dir

flutter start's method of finding devices to run the app on is not suitable for flutter drive.

This commit also refactors several tool services to allow mocking in unit tests.

1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.