Incorrectly resolves absolute source mapping url as relative to file

[rbuckton]

The supportRelativeURL function always assumes mappings are relative, even when they may be an absolute URI.

Given:

• Build 0.3.2 of source-map-support
• a file of c:\dir\a.min.js
• a url of file:///C:/dir/a.js

The result is C:\dir\file:\C:\dir\a.js when it should be C:\dir\a.js.

The issue stems from the fact node's path.resolve function does not interpret file URIs.

[technomage]

This is also the case when source maps contain sourceRoot with a protocol such as cog://NB with file 'Test' should display as cog://NB.Test for the displayed file but instead resolves with local file prefix and drops the // resulting in /working/directory/cog:/NB.Test