internal/imports: update stdlib index for 1.15

    $ go run mkstdlib.go

Updates golang/go#38706.

Change-Id: I9d1bba54c4f9a0369b1d088c00a2f0c3e4409806
Reviewed-on: https://go-review.googlesource.com/c/tools/+/246581
Run-TryBot: Akhil Indurti <[email protected]>
Run-TryBot: Heschi Kreinick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: smasher164

internal/imports/mkstdlib.go

@@ -61,6 +61,7 @@ func main() {
 		mustOpen(api("go1.12.txt")),
 		mustOpen(api("go1.13.txt")),
 		mustOpen(api("go1.14.txt")),
+		mustOpen(api("go1.15.txt")),
 
 		// The API of the syscall/js package needs to be computed explicitly,
 		// because it's not included in the GOROOT/api/go1.*.txt files at this time.