runtime: append([]byte, string...)

Fixes #2274

R=rsc, gri, dsymonds, bradfitz, lvd
CC=golang-dev
https://golang.org/cl/5149045
diff --git a/test/append.go b/test/append.go
index 96421c3..e178f46 100644
--- a/test/append.go
+++ b/test/append.go
@@ -63,6 +63,11 @@
 	{"byte i", append([]byte{0, 1, 2}, []byte{3}...), []byte{0, 1, 2, 3}},
 	{"byte j", append([]byte{0, 1, 2}, []byte{3, 4, 5}...), []byte{0, 1, 2, 3, 4, 5}},
 
+	{"bytestr a", append([]byte{}, "0"...), []byte("0")},
+	{"bytestr b", append([]byte{}, "0123"...), []byte("0123")},
+
+	{"bytestr c", append([]byte("012"), "3"...), []byte("0123")},
+	{"bytestr d", append([]byte("012"), "345"...), []byte("012345")},
 
 	{"int16 a", append([]int16{}), []int16{}},
 	{"int16 b", append([]int16{}, 0), []int16{0}},