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

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.

Stocks has both tabs reified in the wiget tree

PageStorage storage identifier should include the context widget's key

We were recomputing which widgets to show only when we were on the other side
of the repaint boundaries. That doesn't work well for pageable lists because we
come to rest exactly on a repaint boundary, which means we don't cull the other
page.

After this patch, we recompute the set of widgets using an edge-trigger when we
hit the boundary. That's better than using a level-trigger so that we don't
continuously recompute the set of widget as we sit at the boundary.

Switch back to GitHub hosting of stock data

The Firebase server seems to have changed behavior and is not giving us a List
instead of a Map. This patch switches us back to GitHub, which just serves the
same flat files all the time.

Remove flutter_sprites sound_manager.dart

Tweaks near Material, BoxDecoration, implicit animations.

Some more asserts and toString info.
Reorder some code to group lerp-related functions.

clarify that start defaults to checked

Handle degenerate transforms better

No you can't hit things inside degenerate transforms. Tranforming from global
coordaintes to degenerate local coordinates gives you Point.origin. Also,
upgrade vector_math to get a better invert constructor.

Fixes #1161
Fixes #1224

Remove clamp() in favour of double.clamp()

fixes to flutter analyze for the latest dev sdk

don't listen for chromium messages to the adb log

Fixes #1213.

Also, update callers of deprecated methods.

Floating Action Button transitions

When the scaffold's floating action button (FAB) is replaced the old FAB shrinks to zero and then the new one grows to its full size. When a FAB is shown, its child spins into place.

This is all intended to be a good approximation of the "Lateral Screens" section of the material design spec.

The fab.dart example is included as stop gap until the gallery and other demos have been updated.

starter app now has a button that does something

Fix time picker

We were painting the contents of the RRect clip at the wrong offset. This patch
makes RRect match Rect and Path clips.

Fixes #1194

Teach analyze how to print out the total time spent

Makes it more obvious that flutter analyze takes a long time.

@Hixie

Fixes #961

Provide a safe time to update a WidgetToRenderBoxAdapter

If you change the RenderObject tree between frames, you'll assert if
you subsequently hit test. So e.g. if you get two button presses back
to back, and you mutate the tree synchronously in response to the
first one, the second will assert.

This adds an onBuild callback to WidgetToRenderBoxAdapter to make it
easier to do the updates at the right time, i.e., during widget build.
It'll be called whenever you rebuild the WidgetToRenderBoxAdapter
itself, so all you have to do to use it is call setState() on whoever
is building the WidgetToRenderBoxAdapter.

Stop passing bogus bounds to SceneBuilder