go/doc: disable playground for examples that use syscall/js

The playground is not using GOOS=js, so it is not able to use the
package syscall/js. Examples that depend on syscall/js should not
show a "Run" button.

Fixes #28526.

Change-Id: I8b2fcdd0c0ee517a5c3864bf459f813129542389
Reviewed-on: https://go-review.googlesource.com/c/148918
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/doc/example.go

@@ -268,6 +268,11 @@ func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
 		if err != nil {
 			continue
 		}
+		if p == "syscall/js" {
+			// We don't support examples that import syscall/js,
+			// because the package syscall/js is not available in the playground.
+			return nil
+		}
 		n := path.Base(p)
 		if s.Name != nil {
 			n = s.Name.Name