utf8: make EncodeRune's destination the first argument.

R=r
CC=golang-dev
https://golang.org/cl/3364041
diff --git a/src/pkg/bufio/bufio.go b/src/pkg/bufio/bufio.go
index 7d59fb8..4e9f1cf 100644
--- a/src/pkg/bufio/bufio.go
+++ b/src/pkg/bufio/bufio.go
@@ -482,7 +482,7 @@
 			return b.WriteString(string(rune))
 		}
 	}
-	size = utf8.EncodeRune(rune, b.buf[b.n:])
+	size = utf8.EncodeRune(b.buf[b.n:], rune)
 	b.n += size
 	return size, nil
 }