* 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.

* Vertical divider bug fix and additional cleaning.

* Add vertical divider option.

* added vertical divider

* restored hello world

* final changes

* a few changes

* comment fixes

* added VerticalDivider

* few changes

* comment changes:

* ultra nits

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* lint unnecessary_new on samples

* fix tests

This allows other code to get the same style without having to know exactly it is computed.

Fix a bunch of obvious RTL bugs from code inspection.

* Make dividers one device pixel thick as defined in Material design spec

* Updated divider test to check stroke width

* Clarified dividers with 0 height in the docs

* Updated Divider.height docs according to PR feedback

* more widget const constructors

* prefer const constructors

* address review comments

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test

Clarifying that Dividers aren't restricted to usage in lists and Drawers. Maybe no one else is confused about this but I didn't realize it until I found them used in the calculator demo (https://github.com/flutter/flutter/blob/8ca4caa4406046a683c0e2d2a5cf6118fa56fef7/examples/flutter_gallery/lib/demo/calculator/home.dart)

...until people understand them.

Based on observations of usability study participant P1.

This patch completes the circuit on this library. The next step is to go
through and add docs to anything we missed.

Fixes https://github.com/flutter/flutter/issues/3363

The dartdoc will continue until morale improves.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382