Missing source maps for closure_js_test

Hi. First off, I'm not sure if this is related to #16 or #193, so I'm opening a new issue.

I'm finding that for the `closure_js_test` rule in https://github.com/michaelfig/tink/blob/jstest/javascript/BUILD.bazel#L214

when I run the following from the toplevel `tink` directory, it fails with:

```
Michael-Macbook-655:tink michael$ bazel test javascript:test_lib_registry
INFO: Analysed target //javascript:test_lib_registry (0 packages loaded, 0 targets configured).
INFO: Found 1 test target...
FAIL: //javascript:test_lib_registry (see /private/var/tmp/_bazel_michael/6de72905b448fffc7be7652a860
e4346/execroot/tink/bazel-out/darwin-fastbuild/testlogs/javascript/test_lib_registry/test.log)
Target //javascript:test_lib_registry up-to-date:
bazel-bin/javascript/test_lib_registry
INFO: Elapsed time: 5.330s, Critical Path: 5.13s
INFO: 2 processes: 1 darwin-sandbox, 1 local.
INFO: Build completed, 1 test FAILED, 2 total actions
//javascript:test_lib_registry FAILED in 5.1s
/private/var/tmp/_bazel_michael/6de72905b448fffc7be7652a860e4346/execroot/tink/bazel-out/darwin-fastbuild/testlogs/javascript/test_lib_registry/test.log

INFO: Build completed, 1 test FAILED, 2 total actions
```

Okay, no problem, I have a failed test.

What makes things difficult is the lack of source maps, so I get stack traces with something like:
```
exec ${PAGER:-/usr/bin/less} "$0" || exit 1
Executing tests from //javascript:test_lib_registry
-----------------------------------------------------------------------------
Listening http://localhost:57849/index.html
Serving /index.html
Serving /filez/io_bazel_rules_closure/closure/testing/phantomjs_jsunit_runner.js
Serving /filez/tink/javascript/test_lib_registry_bin.js
[...]
01:36:56.134 : Running test: testRegisterPrimitiveWrapper_overwritingWithDifferentClass
-> $module$exports$tink$Registry$registerPrimitiveWrapper$$@http://localhost:53490/filez/tink/javascript/test_lib_registry_bin.js:3084:49
-> testRegisterPrimitiveWrapper_overwritingWithDifferentClass@http://localhost:53490/filez/tink/javascript/test_lib_registry_bin.js:5813:59
[native code]
$$JSCompiler_prototypeAlias$$$$invokeFunction_$$@http://localhost:53490/filez/tink/javascript/test_lib_registry_bin.js:5219:50
$$JSCompiler_prototypeAlias$$$$safeRunTest_$$@http://localhost:53490/filez/tink/javascript/test_lib_registry_bin.js:5206:32
$$JSCompiler_prototypeAlias$$$$safeRunTest_$$@[native code]
$goog$testing$Continuation_$run$$@http://localhost:53490/filez/tink/javascript/test_lib_registry_bin.js:5501:96
```

etc.

I would have thought that rules_closure would make source maps Just Work, so I'm looking for advice on how to enable them, or what we can fix in rules_closure to make this work.