fix(ngMessagesInclude): do not compile template if scope is destroyed

Messages imported with `ngMessagesInclude` are loaded asynchronously and
they can arrive after the ngMessages element has already been removed from DOM.

Previously we tried to compile these messages and that caused a `$compile:ctreq`
error. Now they are silently ignored if `ngMessagesInclude`'s scope has already
been destroyed.

Closes #12695
Closes #14640

src/ngMessages/messages.js

@@ -550,6 +550,8 @@ angular.module('ngMessages', [])
        link: function($scope, element, attrs) {
          var src = attrs.ngMessagesInclude || attrs.src;
          $templateRequest(src).then(function(html) {
+           if ($scope.$$destroyed) return;
+
            $compile(html)($scope, function(contents) {
              element.after(contents);
 

test/ngMessages/messagesSpec.js

@@ -842,6 +842,25 @@ describe('ngMessages', function() {
       })
     );
 
+    it('should not throw if scope has been destroyed when template request is ready',
+      inject(function($rootScope, $httpBackend, $compile) {
+        $httpBackend.expectGET('messages.html').respond('<div ng-message="a">A</div>');
+        $rootScope.show = true;
+        var html =
+            '<div ng-if="show">' +
+              '<div ng-messages="items">' +
+                '<div ng-messages-include="messages.html"></div>' +
+              '</div>' +
+            '</div>';
+
+        element = $compile(html)($rootScope);
+        $rootScope.$digest();
+        $rootScope.show = false;
+        $rootScope.$digest();
+        expect(function() {
+          $httpBackend.flush();
+        }).not.toThrow();
+    }));
   });
 
   describe('when multiple', function() {