[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}

diff --git a/BUILD.gn b/BUILD.gn
index 59d65aa..bc53be2 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -1241,9 +1241,10 @@
     testonly = true
     data_deps = [
       "chrome/browser/resources:closure_compile",
-      "content/browser/resources:closure_compile",
-      "ui/webui/resources:closure_compile",
       "components/ukm/debug:closure_compile",
+      "content/browser/resources:closure_compile",
+      "mojo/public/tools/bindings/generators/js_templates/lite/test:closure_compile",
+      "ui/webui/resources:closure_compile",
     ]
     if (is_chromeos) {
       data_deps += [ "ui/file_manager:closure_compile" ]