runtime, sycall/js: add support for callbacks from JavaScript

This commit adds support for JavaScript callbacks back into
WebAssembly. This is experimental API, just like the rest of the
syscall/js package. The time package now also uses this mechanism
to properly support timers without resorting to a busy loop.

JavaScript code can call into the same entry point multiple times.
The new RUN register is used to keep track of the program's
run state. Possible values are: starting, running, paused and exited.
If no goroutine is ready any more, the scheduler can put the
program into the "paused" state and the WebAssembly code will
stop running. When a callback occurs, the JavaScript code puts
the callback data into a queue and then calls into WebAssembly
to allow the Go code to continue running.

Updates #18892
Updates #25506

Change-Id: Ib8701cfa0536d10d69bd541c85b0e2a754eb54fb
Reviewed-on: https://go-review.googlesource.com/114197
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/js/js_test.go

@@ -7,6 +7,7 @@
 package js_test
 
 import (
+	"fmt"
 	"syscall/js"
 	"testing"
 )
@@ -144,3 +145,52 @@ func TestNew(t *testing.T) {
 		t.Errorf("got %#v, want %#v", got, 42)
 	}
 }
+
+func TestCallback(t *testing.T) {
+	c := make(chan struct{})
+	cb := js.NewCallback(func(args []js.Value) {
+		if got := args[0].Int(); got != 42 {
+			t.Errorf("got %#v, want %#v", got, 42)
+		}
+		c <- struct{}{}
+	})
+	defer cb.Close()
+	js.Global.Call("setTimeout", cb, 0, 42)
+	<-c
+}
+
+func TestEventCallback(t *testing.T) {
+	for _, name := range []string{"preventDefault", "stopPropagation", "stopImmediatePropagation"} {
+		c := make(chan struct{})
+		var flags js.EventCallbackFlag
+		switch name {
+		case "preventDefault":
+			flags = js.PreventDefault
+		case "stopPropagation":
+			flags = js.StopPropagation
+		case "stopImmediatePropagation":
+			flags = js.StopImmediatePropagation
+		}
+		cb := js.NewEventCallback(flags, func(event js.Value) {
+			c <- struct{}{}
+		})
+		defer cb.Close()
+
+		event := js.Global.Call("eval", fmt.Sprintf("({ called: false, %s: function() { this.called = true; } })", name))
+		js.ValueOf(cb).Invoke(event)
+		if !event.Get("called").Bool() {
+			t.Errorf("%s not called", name)
+		}
+
+		<-c
+	}
+}
+
+func ExampleNewCallback() {
+	var cb js.Callback
+	cb = js.NewCallback(func(args []js.Value) {
+		fmt.Println("button clicked")
+		cb.Close() // close the callback if the button will not be clicked again
+	})
+	js.Global.Get("document").Call("getElementById", "myButton").Call("addEventListener", "click", cb)
+}