As of commit 5db46b7, Karma uses version ^0.7.3 of source-map. Version 0.7.0 had introduced several breaking changes. At least one of those breaks error reporting in Karma when source maps are used:

Breaking change: new SourceMapConsumer now returns a Promise object
that resolves to the newly constructed SourceMapConsumer instance, rather
than returning the new instance immediately.

This invariably results in the following error message whenever a test fails:

SourceMap position not found for trace: …