Super Dev Mode: sourcemap URL fixes

Externally visible changes:
- put back old URL. (Needed for some Google integration tests.)
- changed sourcemap filename in URL's to [strong name]_sourcemap.json
  (The '0' is not needed and it doesn't need to match the filename on
 disk.)
- fixed a bug in a regular expression: The period in ".json" was not
  escaped so it would match anything.

Refactoring:
- keep the code for URL-matching separate from the code for finding
sourcemap files on disk.
- Fixed the unit tests to report better errors. Inlined some constants
for readability.

Change-Id: I366b9b13b0026d922428ae3f1ae56abad9d07059
6 files changed