insert type assertions when narrowing.

R=r
OCL=24349
CL=24913
diff --git a/test/fixedbugs/bug054.go b/test/fixedbugs/bug054.go
index 0ed5d07..c8a2272 100644
--- a/test/fixedbugs/bug054.go
+++ b/test/fixedbugs/bug054.go
@@ -23,10 +23,7 @@
 }
 
 func (s *TStruct) field(i int) *TStruct {
-	// works if we say
-	//	t := s.fields.At(i);
-	//	return t;
-	return s.fields.At(i);
+	return s.fields.At(i).(*TStruct);
 }
 
 func main() {