[Mojo Lite] Fix Map for stringable kinds and add Closure Compiler test.

This CL makes stringable key types always return Objects, rather than a
type union with Map, and adds better type information to response
Promises.

It also adds a simple Closure Compiler target to ensure that generated
code can be accessed in certain ways.

Bug: 849993
Change-Id: I6002a577474e45b6b42185988968df8fa4ac6700
Reviewed-on: https://chromium-review.googlesource.com/c/1395870
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#622378}
6 files changed