refactor(ngModelSpec): use valueFn over curry

Refactor ngModelSpec to use internal helper function `valueFn`.
Use instead of multiple-defining a function called `curry`.

PR (angular#15231)

Addresses a quick change mentioned in PR 15208 from Issue angular#14734

test/ng/directive/ngModelSpec.js

@@ -827,21 +827,15 @@ describe('ngModel', function() {
 
 
       it('should only validate to true if all validations are true', function() {
-        var curry = function(v) {
-          return function() {
-            return v;
-          };
-        };
-
         ctrl.$modelValue = undefined;
-        ctrl.$validators.a = curry(true);
-        ctrl.$validators.b = curry(true);
-        ctrl.$validators.c = curry(false);
+        ctrl.$validators.a = valueFn(true);
+        ctrl.$validators.b = valueFn(true);
+        ctrl.$validators.c = valueFn(false);
 
         ctrl.$validate();
         expect(ctrl.$valid).toBe(false);
 
-        ctrl.$validators.c = curry(true);
+        ctrl.$validators.c = valueFn(true);
 
         ctrl.$validate();
         expect(ctrl.$valid).toBe(true);
@@ -875,16 +869,10 @@ describe('ngModel', function() {
 
 
       it('should register invalid validations on the $error object', function() {
-        var curry = function(v) {
-          return function() {
-            return v;
-          };
-        };
-
         ctrl.$modelValue = undefined;
-        ctrl.$validators.unique = curry(false);
-        ctrl.$validators.tooLong = curry(false);
-        ctrl.$validators.notNumeric = curry(true);
+        ctrl.$validators.unique = valueFn(false);
+        ctrl.$validators.tooLong = valueFn(false);
+        ctrl.$validators.notNumeric = valueFn(true);
 
         ctrl.$validate();