playground/internal/imports: Update and regenerate for Go 1.8.

go generate github.com/gopherjs/gopherjs.github.io/playground/internal/imports

Author: dmitshur

playground/internal/imports/mkstdlib.go

@@ -64,6 +64,7 @@ func main() {
 		mustOpen(api("go1.5.txt")),
 		mustOpen(api("go1.6.txt")),
 		mustOpen(api("go1.7.txt")),
+		mustOpen(api("go1.8.txt")),
 		mustOpen("gopherjs.txt"),
 	)
 	sc := bufio.NewScanner(f)