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

R=dgrove@google.com

Review URL: https://codereview.chromium.org//19315003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@25050 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/source_maps/lib/span.dart

diff --git a/pkgs/source_maps/lib/span.dart b/pkgs/source_maps/lib/span.dart
index 7ca0ca9..1d2152d 100644
--- a/pkgs/source_maps/lib/span.dart
+++ b/pkgs/source_maps/lib/span.dart
@@ -69,6 +69,8 @@
   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>';
 }