Fix source map path normalization

We need to normalize the entire path, including the root after it's
made absolute.

lib/src/executable_options.dart

@@ -288,10 +288,10 @@ class ExecutableOptions {
   /// Makes [url] absolute or relative (to the directory containing
   /// [destination]) according to the `source-map-urls` option.
   Uri sourceMapUrl(Uri url, String destination) {
-    var path = p.normalize(p.fromUri(url));
-    return p.toUri(_options['source-map-urls'] == 'relative'
+    var path = p.fromUri(url);
+    return p.toUri(p.normalize(_options['source-map-urls'] == 'relative'
         ? p.relative(path, from: p.dirname(destination))
-        : p.absolute(path));
+        : p.absolute(path)));
   }
 
   /// Returns the value of [name] in [options] if it was explicitly provided by