dart2js_native tests: Migrate 'native STRING' to JS-calls - part 2/2

tests/compiler/dart2js_native/native_named_constructors3_frog_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "native_testing.dart";
 
 // Hidden native class with factory constructors and NO static methods.
 // Regression test.
 
 @Native("A")
 class A {
   // No static methods in this class.
 
   factory A(int len) => makeA(len);
 
   factory A.fromString(String s) => makeA(s.length);
 
-  // Only functions with zero parameters are allowed with "native r'...'".
-  factory A.nativeConstructor() native r'return makeA(102);';
+  factory A.nativeConstructor() {
+    return JS('A|Null', 'makeA(102)');
+  }
 
   foo() native;
 }
 
 makeA(v) native;
 
 void setup() {
   JS('', r"""
 (function(){
   // This code is inside 'setup' and so not accessible from the global scope.
   function A(arg) { this._x = arg; }
   A.prototype.foo = function(){ return this._x; };
   makeA = function(arg) { return new A(arg); };
   self.nativeConstructor(A);
 })()""");
 }
 
 main() {
   nativeTesting();
   setup();
   var a1 = new A(100);
   var a2 = new A.fromString('Hello');
   var a3 = new A.nativeConstructor();
 
   Expect.equals(100, a1.foo());
   Expect.equals(5, a2.foo());
   Expect.equals(102, a3.foo());
 }