perf(md-tooltip) use MutationObserver instead of watcher if available

use watcher on mdVisible only if being used

braces on same line

Author: clshortfuse

src/components/tooltip/tooltip.js

@@ -92,19 +92,42 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $mdUtil, $mdThe
       content.css('transform-origin', origin);
     }
 
+    function onVisibleChanged (isVisible) {
+      if (isVisible) showTooltip();
+      else hideTooltip();
+    }
+
     function configureWatchers () {
       scope.$on('$destroy', function() {
         scope.visible = false;
         element.remove();
         angular.element($window).off('resize', debouncedOnResize);
       });
 
-      scope.$watch('visible', function (isVisible) {
-        if (isVisible) showTooltip();
-        else hideTooltip();
-      });
+      if (element[0] && 'MutationObserver' in $window) {
+        var attributeObserver = new MutationObserver(function(mutations) {
+          mutations
+            .forEach(function (mutation) {              
+              if (mutation.attributeName === 'md-visible') {
+                if (!scope.visibleWatcher)
+                  scope.visibleWatcher = scope.$watch('visible', onVisibleChanged );
+              }
+              if (mutation.attributeName === 'md-direction') {
+                updatePosition(scope.direction);
+              }
+            });
+        });
+
+        attributeObserver.observe(element[0], { attributes: true});
 
-      scope.$watch('direction', updatePosition );
+        if (attr.hasOwnProperty('mdVisible')) // build watcher only if mdVisible is being used
+          scope.visibleWatcher = scope.$watch('visible', onVisibleChanged );
+
+      }
+      else { // MutationObserver not supported
+        scope.visibleWatcher = scope.$watch('visible', onVisibleChanged );
+        scope.$watch('direction', updatePosition );
+      }
     }
 
     function addAriaLabel () {