Introduce ScrollableList2

ScrollableList2 uses the same pattern as ScrollableGrid, which requires the
client to allocate widgets for every list item but doesn't inflate them unless
they're actually needed for the view. It improves on the original
ScrollableList by not requiring a rebuild of the whole visible portion of the
list when scrolling. In fact, small scrolls can often be handled entirely by
repainting.

Hook up wordSpacing and inline height.

rev engine

Add support for scrollable grids

We now support (vertically) scrollable grids with viewporting. If the
scroll doesn't reveal any new rows, we execute the scroll with a repaint
(i.e., no layout). If the scroll reveals a new row, we trigger a layout
to change the set of materialized children in the viewport.

Change how we provide additional information for asserts in the Widget framework.

Fixes #1058.

Generalize grid layout

This patch make grid layout much more flexible. The behavior is factored
out into a GridDelegate that's modeled after the custom layout
delegates. The patch includes a MaxTileWidthGridDelegate that implements
the old behavior and a FixedColumnCountGridDelegate that implements a
grid layout with a fixed number of columns.

Fixes #1048

flutter init should gitignore .atom

RenderBox should use Offset for child offset

Previously we used Position, which makes it harder to accumulate offsets
when walking the render tree.

We now create a .atom folder to hold settings for the Atom project. We
should gitignore that folder our project template.

ScrollbarPainter exception when scrolling MaterialList

When assigning a new overlayPainter, we were detaching the old overlay
painter even if the render object itself wasn't attached. Now we only
twiddle the attach/detach state of the overlay painter when we're
attached ourselves.

Fixes #1047

Ink well in ListItem doesn't fill MaterialList

We need to put the padding on the inside of the list item.

Fixes #1055

update the min api level in the generated android manifest

update the min api level in the generated android manifest; use constants for the android version name

Improve diagnostics around misuse of ParentDataWidgets.

Fixes https://github.com/flutter/flutter/issues/572

tweak the doc/index.html page

Also, trivial improvements to the baseline example that I made while
unsuccessfully trying to reproduce a baseline bug.

Refactor listen in terms of start.

Fixes #1034, at least for start and listen.

Various Input and Focus fixes

Allow travis/test.sh to be run locally (not on travis)

Require a Key on Input.

Simplify the API for Focus.at() and Focus.moveTo().
Fixes #236.
This will require an e-mail to flutter-dev.

Make Input grab focus onTap not onPointerDown.
Fixes #189.

Complain when you use Focus.at() with two different GlobalKeys that
are both in the tree at the same time.
Fixes #181.

Add dartdocs for Focus.moveTo() and Focus.moveScopeTo().

This helps people check that all the tests will pass before they send a PR.

Catch dismissable while it's animating.

Add documentation for GridPaper

I'm not actually sure what this is useful for, but I took a stab at it.

@Hixie

Fixes #173.

RenderFractionalTranslation and Dismissable

- Add RenderFractionalTranslation, a render box that does a
  translation based on a FractionalOffset.

- Make FractionalOffset more like Offset
  - dx/dy instead of x/y
  - add /, ~/, %
  - add .zero

- Add alongOffset and alongSize to FractionalOffset so that you can
  easily apply FractionalOffset to Offsets and Sizes. (Better name
  suggestions welcome.)

- Add transformHitTests boolean to RenderTransform (also on
  RenderFractionalTranslation), and to classes based on it.

- Remove the fade from Dismissable. We can add it back using the
  builder-with-child pattern like Draggable if we need it. See #1003
  for tha feature request.

- Rename a bunch of variables in dismissable.dart.

- Change the test for dismissable to not handle leftwards dismisses
  one pixel different from rightwards dismisses, and cleaned up the
  resulting effect on the test (mostly making sure we had the right
  number of pumps, with comments explaining what each one was).

Fixes #174.

Make TabBarSelection a widget

TabBarSelection is now expected to be an ancestor of its TabBar and TabBarView.