* Use already downloaded dart sdk for frontend_server.
This is follow-up to https://github.com/flutter/flutter/pull/14610
* Remove reference to engine dart-sdk from artifacts
* Remove unused imports
* WIP to switch back to solution where dart sdk is downloaded into bin/cache/dart-sdk, but it is 'flutter precache -a' that can download all dart sdks for other platforms under bin/cache/artifacts/engine/<platform>/dart-sdk
* Download all dart sdks when 'flutter precache -a'
Mainly, this adds documentation to members that were previously
lacking documentation.
It also adds a big block of documentation about improving performance
of widgets.
This also removes some references to package:collection and adds
global setEquals and listEquals methods in foundation that we can use.
(setEquals in particular should be much faster than the
package:collection equivalent, though both should be faster as they
avoid allocating new objects.) All remaining references now qualify
the import so we know what our remaining dependencies are.
Also lots of code reordering in Flutter driver to make the code
consistent and apply the style guide more thoroughly.
