Fixes #8794

There aren't any subclasses of Element that don't also subclass
BuildableElement and I suspect they wouldn't work properly anyway.

Fixes #3656

The new name matches the style guide. (The old name was just old and predated
the style guide.)

Fixes #8000

Covers lib/ in package:flutter.

Fixes #7734

Fixes for: https://github.com/flutter/flutter/issues/7570 https://github.com/flutter/flutter/issues/7231 https://github.com/flutter/flutter/issues/2841 and others

ValueKey and ObjectKey shouldn't be == with subclasses.
Fixes https://github.com/flutter/flutter/issues/3107

Clean up toString for the keys a bit.

Add a test for keys.

The needsLayout getter is prone to misuse. See discussion on #3083.

Rumours have it that this is faster now.

Fixes https://github.com/flutter/flutter/issues/5154

I see these errors in Atom on my Mac. I'm not sure why they're not being
picked up by the bots.

Previously we asserted that the element was not in the dirty list, but there
are scenarios where the element can be in the dirty list already. This patch
makes markNeedsBuild handle those cases by simply resorting the dirty list.

Fixes #6119

Also, make the exception handling for global key listeners slightly more
robust.

...at least not in the traditional markdown style.
So let's try HTML instead.

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

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

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.

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