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