js codegen does not produce helper functions for Any when using closure imports

What version of protobuf and what language are you using?
Version: v3.6.1 (binary downloaded from official releases)
Language: Javascript, when closure styled imports

What operating system (Linux, Windows, ...) and version?

Linux x86_64

What runtime / compiler are you using (e.g., python version or gcc version)
NA

What did you do?
Steps to reproduce the behavior:

root@ec834298ad98:/github/grpc-web-base/third_party/grpc/third_party/protobuf# mkdir -p /tmp/proto_out/closure /tmp/proto_out/commonjs
root@ec834298ad98:/github/grpc-web-base/third_party/grpc/third_party/protobuf# /tmp/bin/protoc -I /github/grpc-web-base/third_party/grpc/third_party/protobuf --js_out=import_style=closure,binary:/tmp/proto_out/closure google/protobuf/any.proto
root@ec834298ad98:/github/grpc-web-base/third_party/grpc/third_party/protobuf# /tmp/bin/protoc -I /github/grpc-web-base/third_party/grpc/third_party/protobuf --js_out=import_style=commonjs,binary:/tmp/proto_out/commonjs/ google/protobuf/any.proto

root@ec834298ad98:/github/grpc-web-base/third_party/grpc/third_party/protobuf# find /tmp/proto_out/ -type f
/tmp/proto_out/commonjs/google/protobuf/any_pb.js
/tmp/proto_out/closure/any.js

root@ec834298ad98:/github/grpc-web-base/third_party/grpc/third_party/protobuf# grep -rHin -C 5  unpack /tmp/proto_out/
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-240-};
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-241-
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-242-
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-243-/**
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-244- * @template T
/tmp/proto_out/commonjs/google/protobuf/any_pb.js:245: * Unpacks this Any into the given message object.
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-246- * @param {function(Uint8Array):T} deserialize Function that will deserialize
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-247- *     the binary data properly.
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-248- * @param {string} name The expected type name of this message object.
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-249- * @return {?T} If the name matched the expected name, returns the deserialized
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-250- *     object, otherwise returns null.
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-251- */
/tmp/proto_out/commonjs/google/protobuf/any_pb.js:252:proto.google.protobuf.Any.prototype.unpack = function(deserialize, name) {
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-253-  if (this.getTypeName() == name) {
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-254-    return deserialize(this.getValue_asU8());
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-255-  } else {
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-256-    return null;
/tmp/proto_out/commonjs/google/protobuf/any_pb.js-257-  }

What did you expect to see
I expect to see pack/unpack functions for closure import code output

What did you see instead?
I do not see pack/unpack. However, when I use commonjs outputs, I see pack/unpack as demonstrated in the repro steps.

Make sure you include information that can help us debug (full error message, exception listing, stack trace, logs).

Anything else we should know about your project / environment

@TeBoring had some investigation into the root cause:

https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/compiler/js/js_generator.cc#L3682
This line is only called in GenerateFile.
https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/compiler/js/js_generator.cc#L3515
When you specify closure style, options.output_mode() == GeneratorOptions::kOneOutputFilePerType, and GenerateFile is not called.

Start in src/google/protobuf/compiler/js/js_generator.cc, especially the referenced GenerateFile and output-mode logic, and compare closure-generated any.js with the commonjs any_pb.js from the reproduction. Done means closure-style output includes the expected Any pack/unpack helpers without regressing commonjs output.