fix(panel): use $event.target instead of $event.srcElement for Firefox compatibility (#11234)

Closes #11035

Author: feloy

src/components/panel/demoGroups/script.js

@@ -72,7 +72,7 @@
       var template = this.menuTemplate;
 
       var position = $mdPanel.newPanelPosition()
-          .relativeTo($event.srcElement)
+          .relativeTo($event.target)
           .addPanelPosition(
             $mdPanel.xPosition.ALIGN_START,
             $mdPanel.yPosition.BELOW
@@ -103,7 +103,7 @@
       var template = this.menuTemplate;
 
       var position = $mdPanel.newPanelPosition()
-          .relativeTo($event.srcElement)
+          .relativeTo($event.target)
           .addPanelPosition(
             $mdPanel.xPosition.ALIGN_START,
             $mdPanel.yPosition.BELOW

src/components/panel/demoPanelProvider/script.js

@@ -85,7 +85,7 @@
       $mdPanel.open('demoPreset', {
         id: 'menu_' + menu.name,
         position: $mdPanel.newPanelPosition()
-            .relativeTo($event.srcElement)
+            .relativeTo($event.target)
             .addPanelPosition(
               $mdPanel.xPosition.ALIGN_START,
               $mdPanel.yPosition.BELOW

src/components/panel/panel.js

@@ -97,7 +97,7 @@ angular
  *       $mdPanel.open('demoPreset', {
  *         id: 'menu_' + menu.name,
  *         position: $mdPanel.newPanelPosition()
- *             .relativeTo($event.srcElement)
+ *             .relativeTo($event.target)
  *             .addPanelPosition(
  *               $mdPanel.xPosition.ALIGN_START,
  *               $mdPanel.yPosition.BELOW