Fix a problem that wasn't found on my mac builds... :-(.

lib/byte_array.cc

Index: lib/byte_array.cc
===================================================================
--- lib/byte_array.cc	(revision 10133)
+++ lib/byte_array.cc	(working copy)
@@ -266,7 +266,7 @@
   intptr_t dst_start_value = dst_start.Value();
   if (length_value < 0) {
     const String& error = String::Handle(String::NewFormatted(
-        "length (%ld) must be non-negative", index));
+        "length (%ld) must be non-negative", length_value));
     GrowableArray<const Object*> args;
     args.Add(&error);
     Exceptions::ThrowByType(Exceptions::kIllegalArgument, args);