Enhancement - Gaurded Type Promotion

[ghost]

Dart supports guarded type promotion - could you guys please consider supporting this in Closure Compiler? I'm sure you're aware of the benefits but I will illustrate it below for anyone reading this who might not know what guarded type promotion is. Shamefully, I was completely unaware of it until recently.

/**
 * @param {Event} event
 */
function listener(event) {
  if (event instanceof MessageEvent) {
    // no type casting is required here because instanceof is ensuring the
    // event is a MessageEvent instance
    console.info(event.data)
  }
}

The following, on the other hand, can get really ugly as we all know.

/**
 * @param {Event} event
 */
function listener(event) {
  if (event instanceof MessageEvent) {
    console.info(/** @type {MessageEvent} */ (event.data))
  }
}

[nicks]

have you tried it? we already do this.

[ghost]

have you tried it? we already do this.

Ah yes, it appears you do. My mistake.

[MatrixFrog]

I knew that we did this but I didn't know the term "guarded type promotion" until now. Cool :)