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

tests/compiler/dart2js_native/native_named_constructors2_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";
 
 // Native class with named constructors and static methods.
 
 @Native("A")
 class A {
   factory A(int len) => _construct(len);
 
   factory A.fromString(String s) => _construct(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)');
+  }
 
   static A _construct(v) {
     return makeA(v);
   }
 
   foo() native;
 }
 
 makeA(v) native;
 
@@ skipping 12 matching lines @@
   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());
 }