https://source.android.com/source/jack.html

Why we'd want to use jack:
- Support for Java 8
- Support for asserts (as well as stripping them in release)
- Faster compiles (well... we'll see anyways :P)
- Potentially more optimized .dex files

It's been shipping with the Android SDK for a while, and lives at: 
//third_party/android_tools/sdk/build-tools/23.0.1/jack.jar
//third_party/android_tools/sdk/build-tools/23.0.1/jill.jar


Note: Adding support for jack would not meen dropping support for javac. We still need javac for:
1. Findbugs, lint, (errorprone?) require .jars
2. Our Eclipse setup uses .jars for our generated files
3. Instrumentation tests use a .jar to extract the test list
4. Robolectric tests

#3 we could certainly find another way around, but the others all seem quite compelling.

Also - we need intermediate .dex files for _incremental targets, but we could produce them from .jack files rather than .jars if we wanted.


Proposed implementation:

Phase 1 (debug):
* Add a use_jack GN arg that would:
  * Invoke jack at the same time as javac when compiling from source
  * Use jack+jill to for android_prebuilt() targets
  * Use jack for final classes.dex assembly

Phase 2 (release):
 * Have our final proguard step use the .jack files rather than the .jar files

For both phases, we'll still use .interface.jar files to know when dependencies have changed meaningfully.