fix(ngAnimateSwap): make it compatible with ngIf on the same element

Previously, both `ngAnimateSwap` and `ngIf` had a priority of 600, which
meant that (while both are [terminal directives][1]) they were executed
on top of each other (essentially messing each other's comment node).

This commit fixes it, by giving `ngAnimateSwap` a priority of 550, which
is lower than `ngIf` but still higher than other directives.

For reference, here is a list of built-in directive per priority:

```
-400: ngInclude, ngView
  -1: ngRef
   1: ngMessage, ngMessageDefault, ngMessageExp, ngModel, select
  10: ngModelOptions
  99: ngHref, ngSrc, ngSrcset
 100: attr interpolation, ngChecked, ngDisabled, ngList, ngMax,
      ngMaxlength, ngMin, ngMinlength, ngModel (aria), ngMultiple,
      ngOpen, ngPattern, ngProp*, ngReadonly, ngRequired, ngSelected,
      ngStep, ngValue, option
 400: ngInclude, ngView
 450: ngInit
 500: ngController
 600: ngAnimateSwap, ngIf
1000: ngNonBindable, ngRepeat
1200: ngSwitchDefault, ngSwitchWhen
```

[1]: https://docs.angularjs.org/api/ng/service/$compile#-terminal-

Fixes #16616

Author: gkalpak

src/ngAnimate/ngAnimateSwap.js

@@ -92,7 +92,8 @@ var ngAnimateSwapDirective = ['$animate', function($animate) {
     restrict: 'A',
     transclude: 'element',
     terminal: true,
-    priority: 600, // we use 600 here to ensure that the directive is caught before others
+    priority: 550, // We use 550 here to ensure that the directive is caught before others,
+                   // but after `ngIf` (at priority 600).
     link: function(scope, $element, attrs, ctrl, $transclude) {
       var previousElement, previousScope;
       scope.$watchCollection(attrs.ngAnimateSwap || attrs['for'], function(value) {

test/ngAnimate/ngAnimateSwapSpec.js

@@ -177,6 +177,29 @@ describe('ngAnimateSwap', function() {
     expect(scopeOne.$$destroyed).toBe(true);
   });
 
+  it('should work with `ngIf` on the same element', function() {
+    var tmpl = '<div><div ng-animate-swap="exp" ng-if="true">{{ exp }}</div></div>';
+    element = $compile(tmpl)($rootScope);
+    $rootScope.$digest();
+
+    var first = element.find('div')[0];
+    expect(first).toBeFalsy();
+
+    $rootScope.exp = 'yes';
+    $rootScope.$digest();
+
+    var second = element.find('div')[0];
+    expect(second.textContent).toBe('yes');
+
+    $rootScope.exp = 'super';
+    $rootScope.$digest();
+
+    var third = element.find('div')[0];
+    expect(third.textContent).toBe('super');
+    expect(third).not.toEqual(second);
+    expect(second.parentNode).toBeFalsy();
+  });
+
 
   describe('animations', function() {
     it('should trigger a leave animation followed by an enter animation upon swap',function() {