Refactor bottom sheet support, add one to the stocks demo

Factored OverlayRoute out of the modal and persistent bottom sheet classes, since the bottom sheet classes need to drive the performance.

Added a bottom sheet to the stocks demo: long-press on a stock shows a modal bottom sheet.

Made AnimatedModalBarrier public.

Factored OverlayRoute out of the modal and persistent bottom sheet clases, since the bottom sheet classes need to drive the performance.

Added a bottom sheet to the stocks demo: long-press on a stock shows a modal bottom sheet.

Made AnimatedModalBarrier public.

test more packages on travis

fix a ws issue in the sample app

Use --devtools-path flag to run_mojo.

Minor doc fixes.

Update instructions for running benchmarks

Add some basic benchmarks of the Stocks app

 - animation_bench.dart. This benchmark measures the full main-thread pipeline
   for ticking the drawer entrance and exit animation.
 - build_bench.dart. This benchmark measures a full app rebuild when there's no
   state change.
 - layout_bench.dart. This benchmark measures a full relayout.

Support for HTTP request body using DataPipeFiller

Search all the build configurations for sky_snapshot

Previously, we assumed the first build configuration would have one. Now we
keep looking until we find one. Also, re-ordered the configurations so that
you'll get the Android one if you have both, which is probably what you would
expect.

Fixes #100

That way we can control if we want to use devtools to run on Linux and
also cleanly separate out the two uses.

R=@jamesr

flutter start should give better errors when it can't find main

Now we suggest using -t to specify the main Dart file.

Fixes #53

Support drag-to-dismiss in persistent bottom sheets

Flinging a persistent bottom sheet downwards dismisses it, per the Material spec.

The showBottomSheet() function now returns a Future, like showModalBottomSheet() does, so that you can discover when it's been dismissed - with navigator.pop() - and with what value.

Factored the drag gesture handling code into _BottomSheetDragController

This CL was flutter/engine#1997

Port test logic to Dart.

Add a CONTRIBUTING.md

This document describes the workflow for contributing code to this repository.

support older android devices (>= 17)

Teach flutter init how to depend on flutter using a relative path

Avoid use of run-as

Issue #1988 results from run-as not working on some phones,
because of Android issue 58373.

This change removes the need for run-as for running
"flutter start". It is still needed for tracing.

The FLX package should test itself

Rather than using the Flutter unit tests to test FLX, we can just test FLX
directly. Also, clean up the dependencies in the pubspec to match the code.