[driver] API for getting performance traces

Example that builds a Flutter Android app using Gradle

Updated AppBar.copyWith parameter names

More flutter analyze regexp cleanup

Add @override to starter template

Runs clean on the repo.  Moving ahead, we can manage exceptions via `.analysis_options` or specific source-level ignores.

Add @override annotations to flutter framework

Improve error handling for ChildView outside MojoShell

Stop using a prebuilt APK

add more type annotations

Instead, require an AndroidManifest.xml and always build an APK.

Fixes #2517

Update .analysis_options to link to blocking bugs

Now we should some placeholder error text in-band in checked mode.

Fixes #2611

[rename fixit] TwoLevelSublist left->leading, center->title

Also, some minor cleanup in activity.dart, though much more is
needed (see https://github.com/flutter/flutter/issues/2668).

Remove some more 'dynamic's

Added AppBarBehavior.under, etc

Even more types

change the description of the drive command

misc tool ui changes

update a field ref in playfair

Execute rename fixit (Part 2)

* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

Removes an LTR bias in DismissDirection.

Fixes #2562

* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)

Fixes #2348

The terms "left" and "right" have an LTR bias. Instead, we now use
"leading" and "trailing", for these list and grid decorations so that
we can later rearrange them to match the reading order.

Fixes #2540
Fixes #2548