This reverts commit 90500a5d.

* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

This converts existing ## Sample code samples to {@tool sample}...{@end-tool} form.

Also:
1. Fixed a minor bug in analyze-sample-code.dart
2. Made the snippet tool only insert descriptions if the description is non-empty.
3. Moved the Card diagram to before the code sample.

This rewrites the sample code analysis script to be a little less of a hack (but still not pretty), and to handle snippets as well.

It also changes the semantics of how sample code is handled: the namespace for the sample code is now limited to the file that it appears in, so some additional "Examples can assume:" blocks were added. The upside of this is that there will be far fewer name collisions.

I fixed the output too: no longer will you get 4000 lines of numbered output with the error at the top and have to grep for the actual problem. It gives the filename and line number of the original location of the code (in the comment in the tree), and prints out the source code on the line that caused the problem along with the error.

For snippets, it prints out the location of the start of the snippet and the source code line that causes the problem. It can't print out the original line, because snippets get formatted when they are written, so the line might not be in the same place.

This re-lands 3c56e6c7 on top of a new Dart SDK that contains new mixin syntax fixes.

This reverts commit 3c56e6c7.

It breaks the AOT build.

* first pass

* revert WidgetsBindingObserver

* disable prefer_mixin

* docs and more mixins

* newer engine; binding fixes

* upgrade dependencies

* fix test binding

* remove whitespace

* remove the obsolete `enableSuperMixins: true`

* upgrade dartdoc to 0.22.0

* temporarily use git version of dartdoc

* fix pub global activate syntax

* use dartdoc 0.23.0

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

It was causing problems rolling Flutter into Fuchsia

* re-enable lint unnecessary_const

* remove trailling whitespaces

* remove unnecessary const (after merge)

This reverts commit cc1cf13e.

For example, mention the icon used for the drawer menu in the docs (this helps people writing unit tests); add DefaultAssetBundle sample code.

* Doc improvements for animations

I got tired of drive-by spelling fixes, so I figured I'd just take care of them all at once.

This only corrects errors in the dartdocs, not regular comments, and I skipped any sample code in the dartdocs. It doesn't touch any identifiers in the dartdocs either. No code changes, just comments.

This prevents some of our mixins from being subclassed.

Also, move mixins to using 'extends' instead of 'implements' for
future compatibility with Dart changes.

Also, rename a class that had Mixin in the name but was not a mixin.

* comprehensive list of lints
* add comments to commented out lint
* fix unnecessary_this lints
* exclude prefer_final_fields

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

* address review comments

Covers lib/ in package:flutter.

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

See #4434.

Doesn't do any of the fancy effects. Just lets the user control the
back transition by sliding from the left, like a drawer. Hero
transitions are disabled during the gesture.

BUG=https://github.com/flutter/flutter/issues/4817

This patch adapts the AppBar to feel more like iOS by centering the title.

Fixes #4962

We should center the title of flexible space bars on iOS.

Related to #4962

As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the

If you tap outside the drop down menu while its animating in, we should
animate it away smoothly. Previously, we jumped to the reverseCurve,
which made the menu disappear immediately. Now we hold the animations as
state, which means we keep their _curveDirection property and don't
switch curves unless the animation actually finishes.

Also, fix a subtle bug in CurvedAnimation whereby we'd never set the
_curveDirection if we didn't see a status change in the parent
animation. Now we initialize _curveDirection based on the current value
of the parent's status.

Fixes #4379