'Return the source map index for minified_src, loading it if not
already loaded.'

def _index_for(self, minified_src):

    if (minified_src not in self._indices):
        for source_dir in self._dirs:
            filename = os.path.join(source_dir, (minified_src + '.map'))
            if os.path.isfile(filename):
                with open(filename) as fp:
                    self._indices[minified_src] = sourcemap.load(fp)
                    break
    return self._indices[minified_src]
