playground/internal/imports: Add Go 1.10 identifiers.

Regenerate with:

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

Author: dmitshur

playground/internal/imports/mkstdlib.go

@@ -66,6 +66,7 @@ func main() {
 		mustOpen(api("go1.7.txt")),
 		mustOpen(api("go1.8.txt")),
 		mustOpen(api("go1.9.txt")),
+		mustOpen(api("go1.10.txt")),
 		mustOpen("gopherjs.txt"),
 	)
 	sc := bufio.NewScanner(f)