Experimental Feature: Add ES6 import style

[Yannic]

This change adds a new import_style that emits ES6 modules.
For now, this only re-exports the symbols from import_style=closure.

In the future, this will no longer require emitting google modules
(goog.provide). If protobuf ever adds support for emitting ES6
modules, we will use them instead of the goog.provided'd versions
as well.

Note that the Bazel integration is currently broken because of a
limitation in rules_closure.

[stanley-cheung]

@Yannic Thanks for the contributions, as always.

There are some build failures. See below. But the bigger issue is that - there is a big change coming from the internal codebase being reviewed at the same time (even coming up the same day!). There was some other issues at hand, which looks like we have to split outputing the normal callback-based client and the Promised-based client into separate files. See #827.

So there will be some conflicts around here: https://github.com/grpc/grpc-web/pull/821/files#diff-2e729f9088e537bfeb6fefc318963a96R1392-R1402. I tried to merge these 2 changes as a test but obviously they have conflicts.

So my plan is to let the internal change finalized first. And then I will export that out (most likely to merge #827), and then can you rebase on top of that? So this may take a while to develop. I will let you know again in this PR when that's ready.

Sorry for the inconvenience.

The build failure:

https://source.cloud.google.com/results/invocations/570b2ff5-c0d5-48db-a494-1d2d25ab8103/targets/grpc%2Fweb%2Fpresubmit/log

Step 16/18 : RUN wget -nv -O buildifier   https://github.com/bazelbuild/buildtools/releases/download/$BUILDIFIER_VERSION/buildifier &&   chmod +x ./buildifier &&   ./buildifier --mode=check --lint=warn --warnings=all -r bazel javascript net &&   rm ./buildifier
 ---> Running in 2e444808854f
2020-05-29 03:30:52 URL:https://github-production-release-asset-2e65be.s3.amazonaws.com/60805080/6e79c580-4910-11ea-9c29-8411a2dd01e4?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWNJYAX4CSVEH53A%2F20200529%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20200529T033052Z&X-Amz-Expires=300&X-Amz-Signature=10db3b8a00be7b5531ad5298830b4fab71ad3590a037788537b3e1c8743f74d5&X-Amz-SignedHeaders=host&actor_id=0&repo_id=60805080&response-content-disposition=attachment%3B%20filename%3Dbuildifier&response-content-type=application%2Foctet-stream [4729152/4729152] -> "buildifier" [1]
bazel/closure_grpc_web_library.bzl:91: uninitialized: Variable "js" may not have been initialized. (https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#uninitialized)
bazel/closure_grpc_web_library.bzl:101: uninitialized: Variable "src" may not have been initialized. (https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#uninitialized)

[Yannic]

Thanks for the update, sounds good. Let me know when the other change landed and I'll rebase and fix this.

[stanley-cheung]

@Yannic Hi, #827 is merged and the corresponding internal change seems to be stable (i.e. not rolled back after a few days). So this should be a safe time to rebase this PR off the latest master now. Thanks!

[Yannic]

@stanley-cheung Thanks, rebased. Fortunately, the merge conflicts turned out to be trivial (both commits inserted a function in the same location). I've also fixed the buildifier warnings.

If this will be mentioned in the release notes, please make sure it's marked as experimental and that we reserve the right to do breaking changes.