fix(scope): watches can be safely unregistered inside watch handlers

Closes angular#2915

Author: scardine

src/ng/rootScope.js

@@ -521,7 +521,7 @@ function $RootScopeProvider(){
                   watch = watchers[length];
                   // Most common watches are on primitives, in which case we can short
                   // circuit it with === operator, only when === fails do we use .equals
-                  if ((value = watch.get(current)) !== (last = watch.last) &&
+                  if (watch && (value = watch.get(current)) !== (last = watch.last) &&
                       !(watch.eq
                           ? equals(value, last)
                           : (typeof value == 'number' && typeof last == 'number'

test/ng/rootScopeSpec.js

@@ -330,6 +330,30 @@ describe('Scope', function() {
       expect(listener).not.toHaveBeenCalled();
     }));
 
+    it('should allow a watch to be unregistered while in a digest', inject(function($rootScope) {
+      var remove1, remove2;
+      $rootScope.$watch('remove', function() {
+        remove1();
+        remove2();
+      });
+      remove1 = $rootScope.$watch('thing', function() {});
+      remove2 = $rootScope.$watch('thing', function() {});
+      expect(function() {
+        $rootScope.$apply('remove = true');
+      }).not.toThrow();
+    }));
+
+    it('should allow a watch to be added while in a digest', inject(function($rootScope) {
+      var watch1 = jasmine.createSpy('watch1'),
+          watch2 = jasmine.createSpy('watch2');
+      $rootScope.$watch('foo', function() {
+        $rootScope.$watch('foo', watch1);
+        $rootScope.$watch('foo', watch2);
+      });
+      $rootScope.$apply('foo = true');
+      expect(watch1).toHaveBeenCalled();
+      expect(watch2).toHaveBeenCalled();
+    }));
 
     it('should not infinitely digest when current value is NaN', inject(function($rootScope) {
       $rootScope.$watch(function() { return NaN;});