* dissallow -release mode on emulators

* have the help text print the default mode

When building AOT, we now run `pub get` instead of printing an error telling
the user to run `pub get`. We can remove this code once the bug reference above
is fixed (should be a couple days).

The app.a wasn't getting pulled into the main executable because we weren't
referencing any of its symbols. Instead, create a dylib that can be packaged
with the application and loaded at runtime.

This patch teaches flutter build aot to build `app.a` for use with the Dart
interpreter.

Currently only android-arm and ios are supported target platforms.

On iOS, we use Xcode to build a static library that contains the precompiled
code. This code is currently unused, but it will be used by the new Xcode
harness to factor out as much complexity as possible into the flutter tool.

Artifacts from local engine builds will be found based on the --local-engine flag

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

@devoncarew @jason-simmons

Also update the engine to a build where these tools have been uplaoded

This currently requires a local build of the engine.

Run "flutter build aot" to build an AOT snapshot.
Run "flutter build apk --profile" to build a "profile mode" APK that uses AOT.