The output from failing unit-test automation in treeherder/phab/try isn't as helpful as that from npm test, particularly the last line:

@Mardak wrote notably last line: @webpack:///test/unit/lib/ActivityStream.test.js:41

TEST-UNEXPECTED-FAIL karma | activity-stream:ActivityStream should exist: expected false to be truthy
AssertionError@http://localhost:9876/base/node_modules/chai/chai.js?db92883971379fbea71a12c25b271c26a3afc3f6:9449:13
[3]</module.exports/Assertion.prototype.assert@http://localhost:9876/base/node_modules/chai/chai.js?db92883971379fbea71a12c25b271c26a3afc3f6:239:13
[5]</module.exports/<@http://localhost:9876/base/node_modules/chai/chai.js?db92883971379fbea71a12c25b271c26a3afc3f6:988:10
propertyGetter@http://localhost:9876/base/node_modules/chai/chai.js?db92883971379fbea71a12c25b271c26a3afc3f6:7899:29
proxyGetter@http://localhost:9876/base/node_modules/chai/chai.js?db92883971379fbea71a12c25b271c26a3afc3f6:9247:22
[6]</module.exports/assert.isOk@http://localhost:9876/base/node_modules/chai/chai.js?db92883971379fbea71a12c25b271c26a3afc3f6:4265:5
@http://localhost:9876/base/test/unit/unit-entry.js?830d4baf862a8aa184a5bdd26fe8a032c108852a:131415:12

vs

  activity-stream
    ActivityStream
      ✖ should exist
        Firefox 68.0.0 (Mac OS X 10.14.0)
      expected false to be truthy
      AssertionError@node_modules/chai/chai.js:9449:13
      [3]</module.exports/Assertion.prototype.assert@node_modules/chai/chai.js:239:13
      [5]</module.exports/<@node_modules/chai/chai.js:988:10
      propertyGetter@node_modules/chai/chai.js:7899:29
      proxyGetter@node_modules/chai/chai.js:9247:22
      [6]</module.exports/assert.isOk@node_modules/chai/chai.js:4265:5
      @webpack:///test/unit/lib/ActivityStream.test.js:41 <- test/unit/unit-entry.js:131415:12

We're just reporting we get from karma-json-reporter. My current guess is that sourcemap information is not being picked up/incorporated by karma-json-reporter. It could also be something in karma-sourcemap-loader, perhaps https://github.com/demerzel3/karma-sourcemap-loader/issues/19