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.go

@@ -43,6 +43,10 @@ func main() {
 	}
 	println("sum(100):", sum)
 
+	// Test select
+	go selectDeadlock()
+	go selectNoOp()
+
 	// Allow goroutines to exit.
 	time.Sleep(time.Microsecond)
 }
@@ -93,3 +97,17 @@ func iterator(ch chan int, top int) {
 	}
 	close(ch)
 }
+
+func selectDeadlock() {
+	println("deadlocking")
+	select {}
+	println("unreachable")
+}
+
+func selectNoOp() {
+	println("select no-op")
+	select {
+	default:
+	}
+	println("after no-op")
+}

testdata/channel.txt

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