* Bump Dart SDK to `1.21.0-dev.6.0`.

* Bump `json_schema` version.

`[IconButton]` is referenced as singular everywhere else, so the _most correct_ version would be singular (`[IconButton]`) with the `s` outside of the brackets.

More idiomatic use of constraints in performResize.

Trivial fixes to comments.

Make ProxyBox not use BoxParentData since it ignores the field.

Make applyPaintTransform more helpful if you use a different ParentData
subclass than RenderBox expects.

Make debugAssertIsValid actually fulfill its contract in RenderObject as
documented.

Add a childBefore for symmetry (we already had childAfter).

Fix the way we dump the child list when there's no children in a
multichild render object.

More asserts in the rendering test library.

...so that when you detach, you still have the slot, the parentData, etc.

I used the wrong identifiers here.

...to avoid confusion with "detachRenderObject" which is basically
unrelated.

We now center the body correctly and handle Fuchsia key events. Also, add
support for modifiers.

See #6861

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