Initial version of Fancy line drawer demo

Part 2 of independent layout pipelines

Adds BuildOwner to manage the dirty list and build processing for
widgets/elements, and adds a widget unit test to make sure separation
is enforced.

Fixes #2723

Rename ToggleControlsDemo to SelectionControlsDemo

Document which widgets require a Material ancestor

Also, fill out other documentation for these widgets.

Fixes #967

Be explicit about the theme in the starter app

These are called selection controls in the spec and in the gallery UI.

This gives developers a breadcrumb they can follow to customize the
theme of their app. Previously, some customers were trying to set a
theme by manually configuring the colors of all their widgets.

Fixes #2980

Polish Slider implementation

Snackbar opacity animation shouldn't trigger on reverse

Based on feedback from the material design team.

Fixes #1613

Tab updates

Fixes #617

Update engine

Make it possible to dismiss a snackbar manually

Fixes #463

This patch updates our use of Mozart to the latest version of the protocol.

add services to schema

Fixes the broken Weather demo

DatePicker et al updates

These height values are incorrect

They're actually leading values rather than line height values, which means
they're operative between paragraphs not within a given paragraph.

fix image loading

Improve docs for TextStyle#height

Fixes #2918

Update engine

rev mockito; remove deprecated uses

Also, pass font information to ui.ParagraphStyle.

run pub with --no-package-symlinks

The intrinsic size checks were very slow