Incorrect resolution of absolute paths

[jfirebaugh]

Say I have a source file with URL https://www.example.com/path/file.js with the following sourcemap directive:

//# sourceMappingURL=/path/file.js.map

Expected behavior: https://www.example.com/path/file.js.map is requested.
Actual behavior: https://www.example.com/path//path/file.js.map is requested, which fails.