sky/engine and sky/services updates for Mac target

Build configuration and toolchain updates for iOS and Mac targets

Measurement tracking for fitness app.

There is an issue with the input control that prevents actually creating a measurement. Working on this with eseidel.

R=eseidel

Fix gn --ios

Currently crashes with a python exception due to typo.

R=abarth@google.com

Card Collection dismiss animation

Fix Release builds

Fix typo "natvies" in the build system.

Remove focus support from sky/engine/core/*.

tabindex, :focus, actual focus support, etc.

Tweaks to launcher

Remove shadow trees and custom elements from sky/engine/.

Try building Release with -j 2

Remove all code relating to shadow trees, insertion points, shadow
boundaries, traversing composed trees, distribution, template
documents, custom elements, registering elements, element registries,
element factories, shadow roots, etc.

Remove the following features from the IDLs and from the binding
generators: CustomElementCallbacks, Reflect*, EventHandler.

Remove the CSS custom pseudo-element concept, since we no longer have
a UA style sheet worth talking about, no longer have shadow trees or
custom elements, no longer use pseudo-elements, and generally
therefore don't use this code at all.

Maybe this will work now that we have less code.

Update to mojo 5561fc7ae1717f8013a873f744fdae116d046e74

Use "org.domokit" in Sky shell iOS

Roll mojo

Updated CONTRIBUTING.md

Draw debug rects when flex overflows and debugPaintSizeEnabled

Conflicts:
	sky/sdk/lib/rendering/flex.dart

These directories aren't referenced anymore.

Support for non-alphabetic baselines