helloworld_dev_server and helloworld_dev don't generate source maps

[johnthuss]

Describe the bug
No source map is generated when running the helloworld_dev_server task or the helloworld_dev task.

To Reproduce
cd samples/helloworld
ibazel run src/main/java/com/google/j2cl/samples/helloworld:helloworld_dev_server

Navigating to the source map's url (http://localhost:6006/HelloWorld.js.map) shows this error message instead of a source map:
"No srcs found in transitive closure with path component prefix matching request path."

Similarly, a dev build using bazel build src/main/java/com/google/j2cl/samples/helloworld:helloworld_dev generates an empty file for the source map:
bazel-bin/src/main/java/com/google/j2cl/samples/helloworld/helloworld_dev.js.map

Bazel version
Build label: 1.1.0
Build target: bazel-out/darwin-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Mon Oct 21 08:47:13 2019 (1571647633)
Build timestamp: 1571647633
Build timestamp as int: 1571647633

Expected behavior
I'd like to have source maps for my J2CL projects during development to aid in debugging. Without this, it is a step backwards from GWT 2.

[gkdn]

Thanks for the detailed report.

This is actually already tracked in #10 so I will close this one.