Use the new "offset" name in span.dart too.

Review URL: https://codereview.chromium.org//397343005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_maps@38376 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/lib/span.dart b/lib/span.dart
index 7b513e6..b546a97 100644
--- a/lib/span.dart
+++ b/lib/span.dart
@@ -390,5 +390,5 @@
   SpanFormatException(String message, Span span, [this.source])
       : super(message, span);
 
-  int get position => span == null ? null : span.start.offset;
+  int get offset => span == null ? null : span.start.offset;
 }