From a chat with Jinseong:
Previously, without providing any configuration, R8 works like normal dexer if its target is set as DEX backend, keeping everything. After this: https://r8-review.googlesource.com/c/r8/+/32300 you need to specify those rules mentioned in that CL to get what you expect: running r8 as a normal dexer

Using R8 as the dexer might require us passing it a full classpath, which previously wasn't required for D8. If we don't pass it a full classpath, we get a lot of these type of warnings:

Warning: Missing class: java.lang.ref.WeakReference
Warning: Missing class: java.lang.Object

 Issue 883166  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/70dbeca1e052eb492c7a7e478ebe5676f520f050

commit 70dbeca1e052eb492c7a7e478ebe5676f520f050
Author: Sam Maier <smaier@chromium.org>
Date: Fri Jan 04 20:13:14 2019

Making javapoet not supports_android

R8 is having issues dexing this target, which it shouldn't try to do in
the first place.

Bug: 917458
Change-Id: Ic95c9419f5bae9255700cfc34b1779b46844b47d
Reviewed-on: https://chromium-review.googlesource.com/c/1393665
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620042}
[modify] https://crrev.com/70dbeca1e052eb492c7a7e478ebe5676f520f050/third_party/android_deps/BUILD.gn
[modify] https://crrev.com/70dbeca1e052eb492c7a7e478ebe5676f520f050/tools/android/roll/android_deps/build.gradle

This issue has an owner, a component and a priority, but is still listed as untriaged or unconfirmed. By definition, this bug is triaged. Changing status to "assigned". Please reach out to me if you disagree with how I've done this.