replace non-blocking send, receive syntax with select

R=golang-dev, nigeltao, niemeyer, r
CC=golang-dev
https://golang.org/cl/4079053
diff --git a/test/errchk b/test/errchk
index b0edd7a..fbb021c 100755
--- a/test/errchk
+++ b/test/errchk
@@ -73,6 +73,7 @@
 	my @match;
 	foreach my $src (@{$src{$file}}) {
 		$line++;
+		next if $src =~ m|////|;  # double comment disables ERROR
 		next unless $src =~ m|// (GC_)?ERROR (.*)|;
 		$regexp = $2;
 		if($regexp !~ /^"([^"]*)"/) {