fix($location): back-button should fire $locationChangeStart

Before $locationChangeStart event is not broadcast when pressing the back-button on the browser.

Closes #2109

Author: quazzie

src/ng/location.js

@@ -577,6 +577,10 @@ function $LocationProvider(){
     // update $location when $browser url changes
     $browser.onUrlChange(function(newUrl) {
       if ($location.absUrl() != newUrl) {
+        if ($rootScope.$broadcast('$locationChangeStart', newUrl, $location.absUrl()).defaultPrevented) {
+          $browser.url($location.absUrl());
+          return;
+        }
         $rootScope.$evalAsync(function() {
           var oldUrl = $location.absUrl();
 

test/ng/locationSpec.js

@@ -1216,7 +1216,7 @@ describe('$location', function() {
         expect($location.url()).toEqual('');
 
         $rootScope.$on('$locationChangeStart', function(event, newUrl, oldUrl) {
-          throw Error('there is no before when user enters URL directly to browser');
+          $log.info('start', newUrl, oldUrl);
         });
         $rootScope.$on('$locationChangeSuccess', function(event, newUrl, oldUrl) {
           $log.info('after', newUrl, oldUrl);
@@ -1226,6 +1226,8 @@ describe('$location', function() {
         $browser.url('http://server/#/somePath');
         $browser.poll();
 
+        expect($log.info.logs.shift()).
+          toEqual(['start', 'http://server/#/somePath', 'http://server/']);
         expect($log.info.logs.shift()).
           toEqual(['after', 'http://server/#/somePath', 'http://server/']);
       })