Introduce Tween and the new animation API

This patch removes state from the animation system, which was causing problems
as we were scaling the use of animated values.

Now the "tween" objects are stateless and can watch animations, which creates a
new object that holds both the tween and the animation instead of mutating the
tween every tick of the animation.

This patch ports one client as a proof-of-concept.

Fixes #215

fix two null-aware-operator issues

StackParentData.toString() fix, RenderOpacity default

Move RenderObjectWidgets down

It's confusing to have non-render-object widgets on either side of the
render object ones.

Fix some intrinsic constraints contract violations.

RenderBlock wasn't constraining the results.
RenderPadding wasn't constraining the results (which matters
especially when the constraints can't fit the padding in the first
place).
RenderViewport wasn't constraining the results.

Add a test for the block case.

To catch this kind of thing in the future, add some asserts to
debugDoesMeetConstraints() that all four intrinsic functions return
values that are within the constraints.

RenderBlockViewport doesn't support returning intrinsics, so turn off
the "no intrinsic support" asserts (and return zero) when we're doing
this new assert.

This new assert screwed up the custom layout classes' tests, so adjust
those tests to ignore the callbacks invoked from these asserts.

Add to the _debugReportException() method a short summary of the
descendants of this node. It's important to have this information when
debugging errors like these intrinsic constraints contract violations
because often nodes just pass the values through to their child so you
have to go several steps down to find the actual problem.

Fixes https://github.com/flutter/flutter/issues/1210

More elaborate RenderBox example

Reimplement the theme transition animation by actually animating the Theme

As part of this:
 - A lot of classes got new lerp functions, including e.g. TextStyle.
 - Theme's constructor story got overhauled. You can now configure
   everything if you really want to, and we're better about defaults.
 - Material no longer automatically animates its background color.
   (It still does for its shadow.)
 - Tabs try to get the indicator color from the theme.
 - The fields in ThemeData got reordered for sanity.
 - Theme.== and Theme.hashCode got fixed.
 - Typography got a bit of a spring cleaning.

Fixes #613.

Skip "list of devices" header from adb

Sometimes "adb devices" prints a header. We should skip over the header
instead of complaining that we don't recognize it.

Fixes #1293

Rename ViewConstraints to ViewConfiguration

Rename _Child to _ScaffoldSlot for clarity.

Make 'flutter analyze' support analyzing arbitrary files

Activity clean-up

Also, some trivial fixes for things that I found while playing with
the rendering library directly.

Include BoxDecoration description in Container toString

Prepare flutter analyze for dart 1.14

And rootConstraints to configuration.

This type and its variable have nothing to do with Constraints.

Fixes https://github.com/flutter/flutter/issues/829

Before we didn't know how to find the packages of random files. Now we do.

Turns out Android crashes if the colour is not opaque, so we enforce
that at the Dart level.

Also, since label and colour are both actually optional, make them
named arguments.

fix framework to match new mojo sdk

The dart bindings changed semantics in a non-compatible way.

Remove Firebase flutter packge

fix links to new website

Update progress_indicator widget example

Update the progress_indicator demo: it was using Title explicitly, which is no longer needed.

Defend against a null color in updateTaskDescription. The color field in the TaskDescription struct (activity.mojom) isn't nullable.

Fixes #1248

Have BoxDecoration default to one-line dumps

Style nits in a rendering example

Tooltips

Introduces a new Tooltip class.
Adds support for tooltips to IconButton and Scaffold.
Adds some tooltips to various demos.

Also some tweaks to stack.dart that I made before I decided not to go
down a "CustomPositioned" route.