Currently, Blink has no way to mock service connectors client-side in JS for layout tests. This is blocking the port of several Mojo interfaces to be hosted in the Device Service rather than //content/browser, as layout tests rely on providing mock implementations by calling interfaceProvider.addOverrideForTesting().

This email thread has more context: https://groups.google.com/a/chromium.org/forum/#!topic/platform-architecture-dev/RGIi1FxI8CE.

This CL is an attempt to add this functionality: https://codereview.chromium.org/2643063002/. It is currently stalled.