`flutter start` crashes when port 8181 is in use

Catch the error and log a warning.

Fixes #1050

TabBarView material gallery demo

Rationalise all our exception catching and reporting code

- make them all have the same style
- make them all include the stack trace last
- make them all stop printing if their callback is set (if
  appropriate, they don't yet all have callbacks)

Remove unreferenced getter "debugDoesLayoutWithCallback".

I think I wanted to use this at some point but ended up going a
different direction. Instead, object.dart has a private field that
serves a similar purpose.

AnimatedContainer has an issue with one config value change and not another

Revert "Make `flutter listen` exit nicely when missing dependencies"

Make `flutter listen` exit nicely when missing dependencies

If we're already at the target value, we fail to configure the variable. If
another variable animates, we re-animate the other variable.

Fixes #958

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.