fix($parse): correctly assign expressions who's path is undefined and that use brackets notation

Closes #8039

test/ng/parseSpec.js

@@ -1089,6 +1089,22 @@ describe('parser', function() {
             fn.assign(scope, 123);
             expect(scope).toEqual({a:123});
           }));
+
+          it('should expose working assignment function for expressions ending with brackets', inject(function($parse) {
+            var fn = $parse('a.b["c"]');
+            expect(fn.assign).toBeTruthy();
+            var scope = {};
+            fn.assign(scope, 123);
+            expect(scope.a.b.c).toEqual(123);
+          }));
+
+          it('should expose working assignment function for expressions with brackets in the middle', inject(function($parse) {
+            var fn = $parse('a["b"].c');
+            expect(fn.assign).toBeTruthy();
+            var scope = {};
+            fn.assign(scope, 123);
+            expect(scope.a.b.c).toEqual(123);
+          }));
         });