Be explicit about the theme in the starter app

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

Now we run them once per render object.

Fixes #2487

Reactivating a StatefulElement should imply a build

Fixes #2901

More table fixes

Animate TwoLevelList leading icon color

Add keys to ResourceImages.

[driver] fix observatory port # and timeline data extraction