See https://github.com/flutter/flutter/pull/6861

This silences all but two of the warnings from https://travis-ci.org/flutter/flutter/builds/176055550

One of the silenced warnings was a genuine code error.

Some of the others were correct but there was no way for the analyzer to know, and I worked around them.

The remainder are problems with the analyzer, specifically https://github.com/dart-lang/sdk/issues/27827 and https://github.com/dart-lang/sdk/issues/27504.

Some minor tweaks to answer questions I had when looking at this.

See #6631

* 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

Also fixes https://github.com/flutter/flutter/issues/6510

* convert pubGet to throw ToolExit on non-zero exit code
* convert commandValidator to throw ToolExit for non-zero exit code
* convert flutter commands to throw ToolExit for non-zero exit code
* use convenience method throwToolExit
* only show "if this problem persists" for unusual exceptions

This should help a bit with people trying to make apps that have a
Material feel but don't necessarily exactly fit the Material spec.

Exception in Flutter Gallery / Dialogs Demo #6826

* convert flutter commands to use ToolExit for non-zero exit code
* add convenience method throwToolExit
* print ToolExit message iff not null

The added printTrace statements are causing problems when running tests locally on Linux.
This reverts commit 546cef52.

These define a TextTheme and IconTheme that contrast with the accent
colour brightness. Also adjust default accentColorBrightness to match
Material spec examples (dark text/icons on teal in Dark theme).

Update material components to use accentTextTheme, accentIconTheme:
* DatePicker selection
* Floating action button icon
* TimePicker selection
* Slider label text

- [x] Return the reload message even when paused at a breakpoint.
- [x] Only initialize context members if they aren't already initialized.

Fixes https://github.com/flutter/flutter/issues/6690

Also add a version of the Gallery smoke test that enables semantics

- [x] Stop tracking time to first frame because it is unreliable.
- [x] Track time to complete reassemble instead.

I was in here trying to figure out why we rebuild sky_services (the
answer is, because we rebuild every package we've ever downloaded, but
that's not really a problem for new users so whatever), and while I
was here I did some cleanup.

- [x] Remove old benchmark code from the non-hot runner.
- [x] Remove some dead code.
- [x] Fix a throw that should be a rethrow.