fixes #16
diff --git a/lib/builder.dart b/lib/builder.dart
index 091e220..1f36b99 100644
--- a/lib/builder.dart
+++ b/lib/builder.dart
@@ -77,7 +77,8 @@
/// location in the target file. We sort primarily by the target offset
/// because source map files are encoded by printing each mapping in order as
/// they appear in the target file.
- int compareTo(Entry other) {
+ int compareTo(_other) {
+ Entry other = _other as Entry;
int res = target.compareTo(other.target);
if (res != 0) return res;
res = source.sourceUrl.toString().compareTo(
