fix(mdTooltip): Tooltip parent aria label override (#10464)

The tooltip now does not override or update the parent
element `aria-label` unless the parent's `aria-label` does not exist,
or it has been previously set by the tooltip. It determines if it
has been set by the tooltip by placing the `aria-labelledby` attribute
on the parent element and setting it to `tooltip`.

Fixes #10242.

Author: ThomasBurleson

src/components/tooltip/tooltip.js

@@ -75,6 +75,7 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $interpolate,
 
   function linkFunc(scope, element, attr) {
     // Set constants.
+    var tooltipId = 'md-tooltip-' + $mdUtil.nextUid();
     var parent = $mdUtil.getParentWithPointerEvents(element);
     var debouncedOnResize = $$rAF.throttle(updatePosition);
     var mouseActive = false;
@@ -102,12 +103,24 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $interpolate,
       }
     }
 
-    function addAriaLabel(override) {
-      if (override || !parent.attr('aria-label')) {
-        // Only interpolate the text from the HTML element because otherwise the custom text
-        // could be interpolated twice and cause XSS violations.
-        var interpolatedText = override || $interpolate(element.text().trim())(scope.$parent);
+    function addAriaLabel(labelText) {
+      // Only interpolate the text from the HTML element because otherwise the custom text could
+      // be interpolated twice and cause XSS violations.
+      var interpolatedText = labelText || $interpolate(element.text().trim())(parent.scope);
+
+      // Only add the `aria-label` to the parent if there isn't already one, if there isn't an
+      // already present `aria-labelledby`, or if the previous `aria-label` was added by the
+      // tooltip directive.
+      if (
+        (!parent.attr('aria-label') && !parent.attr('aria-labelledby')) ||
+        parent.attr('aria-labelledby') === tooltipId
+      ) {
         parent.attr('aria-label', interpolatedText);
+
+        // Set the `aria-labelledby` attribute if it has not already been set.
+        if (!parent.attr('aria-labelledby')) {
+          parent.attr('aria-labelledby', tooltipId);
+        }
       }
     }
 
@@ -360,7 +373,6 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $interpolate,
       }
 
       if (!panelRef) {
-        var id = 'tooltip-' + $mdUtil.nextUid();
         var attachTo = angular.element(document.body);
         var panelAnimation = $mdPanel.newPanelAnimation()
             .openFrom(parent)
@@ -371,7 +383,7 @@ function MdTooltipDirective($timeout, $window, $$rAF, $document, $interpolate,
             });
 
         var panelConfig = {
-          id: id,
+          id: tooltipId,
           attachTo: attachTo,
           contentElement: element,
           propagateContainerEvents: true,