Address hint discovered by dart2js: add hashCode to source_maps.Span

pkg/source_maps/lib/span.dart

Index: pkg/source_maps/lib/span.dart
diff --git a/pkg/source_maps/lib/span.dart b/pkg/source_maps/lib/span.dart
index 7ca0ca97c4cc13377205e05cd19c8f96d90e4fb6..1d2152da70c4e59c7ff0a1b58d9dcac0f8c28af6 100644
--- a/pkg/source_maps/lib/span.dart
+++ b/pkg/source_maps/lib/span.dart
@@ -69,6 +69,8 @@ abstract class Span implements Comparable {
   bool operator ==(Span other) =>
     sourceUrl == other.sourceUrl && start == other.start && end == other.end;
 
+  int get hashCode => sourceUrl.hashCode + start + (31 * (end - start));
+
   String toString() => '<$runtimeType: $start $end $formatLocation $text>';
 }