Fixes https://github.com/flutter/flutter/issues/6849

* add @required when there's an assert not null

* address review comments

* [devFS] Use URI to represent paths on device

Previosuly, regular file paths in the format of the host platform were used to represent paths on device. That works when host and device share the same (POSIX) file path format. With a Windows host, this breaks. URIs are the solution as they are platform independent and the VM service on the device already interpreted the file paths as URIs anyways.

* review comments

* switch to file paths

* fix tests on Windows

* review comments

Turns out when I added tests, I broke the actual underlying functionality.

* Small Flutter strong mode cleanup fixes.

These are cases where strong mode down cast composite errors
generally indicated cases that would performance or correctness
issues if Flutter code was run in a strong mode VM.

* Fix Command API so that it is always in terms of Map<String,String>.

* Fix typedef

This allows us, for example, to wait for the slow mode banner to have
been removed from the screen before triggering a screen shot.

Add an "s" command to `flutter run` which takes a screenshot.
Make that comment turn off the slow mode banner.

* Return null from VM.mainView if no view exists
* Retry in connectToServiceProtocol if a view is not yet available
* Do not explicitly call exit from the benchmarks - it will not cleanly shut down the engine

...now that we have generic methods, their types need to be specified too.

* relax timeouts for some service protocol calls

* remove 'note that' text

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

The default rpc.Peer constructor was applying a transformer that filters
out these errors.

See https://github.com/flutter/flutter/issues/7174

* reapply handle new Observatory URL changes
Fixes https://github.com/flutter/flutter/issues/6843

* allow non numeric IPv4 addresses
Fixes https://github.com/flutter/flutter/issues/7051

https://github.com/flutter/flutter/commit/737a55ef3e47036220e72f3b3e17353d461b5fab

- [x] Remove the two stage initial boot process.
- [x] Remove the loader screen app.
- [x] Don't report reload times for the initial reload because we are
switching from a snapshot to source and that will always be worst case.

* add a pause parameter to the reload command

* encode OperationResult as json

* reverse isOk logic

This patch adds `@checked` everywhere is needed to remove the
`strong_mode_invalid_method_override` strong mode error.

* Udpate to Dart `1.20.0-dev.1.0`.

Udpate to Dart `1.20.0-dev.1.0` and corresponding `analyzer` package.

* Added `--no-packages-dir`.

* Revert extension to master.

* Added ignores.

* Review nits.

* More nits.