[ios] reduce xcode version requirement to 7.0

Update engine

Add DevicePortForwarder

Build bots only have 7.0.

Add an option to specify a working directory for the FLX builder

[ios] look for the best available devicetype/runtime when booting

Add a basic Text Fields demo to Material Gallery app.

BUG=https://github.com/flutter/flutter/issues/1547

`flutter run` should fail if `pub get` fails.

Previously we were ignoring the return code and continuing.

@devoncarew

Scaffold saves and restores scrollable state, etc

Update engine

Fixes #2569

Enable always_declare_return_types lint

And fix a zillion omissions this uncovered.

[engine] bump version to f2d8d75897a5aadcd9be0067a78fce1cea677fd6

Make flutter analyze useful for package conflicts

Also, resolve our package conflict, since reflectable has been fixed.

Not as

It has performance implications in release mode.

We'll enable the lint once https://github.com/dart-lang/linter/issues/195 is fixed.

This allows building an FLX without leaving any temporary files in the app's
source directory

Improve InputValue#toString

Test harness should check for running Timers and AnimationControllers

After running a widget test, we now clear out the widget tree and check that we
didn't leak any timers or animations.

Also, fix several bugs that this addtional check revealed.

Fixes #2481

Adds initial version of Markdown renderer

Add `flutter_tools build` hooks to also generate depfile.

fix a regression in flutter logs

Fixes #1942.

Use lowerBound() from package:collection/collection.dart

Added SizeTransition