Make `flutter listen` exit nicely when missing dependencies

Previously it died with a confusing exception.

@chinmaygarde

Enable more asserts when building with virtual viewports

Switch PageableList over to using RenderList

This patch moves PageableList off HomogeneousViewport and onto RenderList and
friends, making it match the new ScrollableList.

Download the Xcode artifacts for the current engine revision

have flutter logs log stderr

Teach `flutter trace stop` to use time on the device.

This is yet another hack on top of adb log handling.
This is fragile, but w/o this if you device time is out
of sync, you can't trace.

@abarth

Replace ScrollableList with ScrollableList2

ScrollableList2 is complete and subsumes all the use cases for the original
ScrollableList.

Convert TabBar to TabBar<T> and TabBarSelection to TabBarSelection<T>

The TabBarSelection constructor no longer has the odd maxIndex parameter and the selection is initialized by value rather than the index of the selected tab.

TabBar has a Map labels parameter instead of List.

Fix assert in Stocks app

When paging a scrollable list, we were trying to read the render object's size
at a time when we're not allowed to read it. Instead, encode the information
into the repaint limit, which is more correct (and faster) anyway.

Complete features of ScrollableList2

This patch also changed ScrollableList2 to use an Iterable instead of an
List for its children. This change lets clients map their underlying
data lazily. If the clients actually have a concrete list, we skip the
extra copy and grab the child list directly.

This patch implements the remaining missing features of ScrollableList2.
It should now be nearly a drop-in replacement for ScrollableList. The
next patch will switch callers over to the new machinery.

clear logs on app launch

Remove ScrollDirection.both

Add flags for signing an APK with a custom keystore

Almost none of the clients of ScrollDirection can handle scrolling in both
directions.

Fixes #151

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

Fixes #1010

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