tooltip aria watchers

[Head]

Pullrequest 7822 https://github.com/angular/material/pull/7822/files was doing such a good work getting rid of many watchers and then there were added the following code that again adds thousand of watchers for my application:

// Updates the aria-label when the element text changes. This watch
// doesn't need to be set up if the element doesn't have any data
// bindings.
if (element.text().indexOf($interpolate.startSymbol()) > -1) {
    scope.$watch(function() {
      return element.text().trim();
    }, addAriaLabel);
}

[clshortfuse]

Since I wrote the original PR to improve performance, I'll take a look at this regression.

[clshortfuse]

Culprit was a rebase/timing issue when moving to mdPanel. Should be fixed soon.