Fix type checking of void type.

runtime/bin/mime_multipart_parser.dart

Index: runtime/bin/mime_multipart_parser.dart
===================================================================
--- runtime/bin/mime_multipart_parser.dart	(revision 9649)
+++ runtime/bin/mime_multipart_parser.dart	(working copy)
@@ -53,7 +53,7 @@
     _headerValue = new StringBuffer();
   }
 
-  void update(List<int> buffer, int offset, int count) {
+  int update(List<int> buffer, int offset, int count) {
     // Current index in the data buffer. If index is negative then it
     // is the index into the artificial prefix of the boundary string.
     int index;