Fix error in processing dart-ext urls with a relative path.

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index 6c423a95db1dc3369eb37d47f85fc502275ac5fc..cb3691aa8d7763703b4d5fe4f0177b70de113c1c 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -244,7 +244,11 @@ String _filePathFromUri(String userUri) {
       return uri.toFilePath();
       break;
     case 'dart-ext':
-      return new Uri.file(uri.path).toFilePath();
+      // Relative file URIs don't start with file:///.
+      var scheme = (uri.path.startsWith('/') ? 'file' : '');
+      return new Uri(scheme: scheme,
+                     host: uri.host,
+                     path: uri.path).toFilePath();
       break;
     case 'package':
       return _filePathFromPackageUri(uri);