all: implement trivial select statements

Implement two trivial uses of the select statement.

Always blocking:
    select {}

No-op:
    select {
    default:
    }

Go 1.12 added a `select {}` instruction to syscall/js, so this is needed
for Go 1.12 support. More complete support for select will be added in
the future.

testdata/channel.txt

@@ -19,3 +19,6 @@ sum: 25
 sum: 29
 sum: 33
 sum(100): 4950
+deadlocking
+select no-op
+after no-op