Use FractionalOffset for gradients

Rename AnimatedWidgetBase -> ImplicitlyAnimatedWidget

These were using Offsets, but they're really FractionalOffsets.

Fixes #2318

The previous name was confusingly similar to AnimatedWidget, which is
for explicit animations.

Tapping drawer during animation causes it to stick

The problem was we were using a tap gesture to stop the motion of the
drawer and a drag gesture to settle it. That can cause a broken
lifecycle. Now we use a single drag recognizer to drive the whole
lifecycle.

Fixes #775
Fixes #1276

Change SizeTransition API to take a double instead of FractionalOffset for alignment

Improve dartdoc for GlobalKey

Fixes #2708

Update CONTRIBUTING.md

Update Engine Version to 5e68565d713cbb89e0d60e4c635cf7a9e4b2b855

Provide a list of Material design primary and accent color swatches

minor update to flutter daemon

Let RenderObjectWidgets use inherited properties

Update RichText dartdoc

[ios] improve test device naming

The new name format is "DEVICE_TYPE_NAME (Flutter)". For example:

iPhone 6 (Flutter)
iPhone 6s Plus (Flutter)

The names are taken from `xcrun simctl list --json devicetypes`.

Fixes #2682

The old text was misleading because it was written when RichText was an
implementation detail of Text.

Fixes #2624

This prepares us for depending on the LTR/RTL inherited property in
RenderObjectWidgets.

[driver] API for getting performance traces

Example that builds a Flutter Android app using Gradle

Updated AppBar.copyWith parameter names

More flutter analyze regexp cleanup

Add @override to starter template

Runs clean on the repo.  Moving ahead, we can manage exceptions via `.analysis_options` or specific source-level ignores.

Add @override annotations to flutter framework

Improve error handling for ChildView outside MojoShell

Stop using a prebuilt APK

add more type annotations