Move tests using frog 'native' extension from tests/language to frog/tests

tests/language/src/NativeNamedConstructors1FrogTest.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.
-
-#library('X');
-#native('NativeNamedConstructors1FrogTest.js');  // Defines JS constructor 'A'.
-
-// The native class has several constructors which partition the behaviour of
-// the JS constructor function into several well-typed Dart constructors.
-
-class A native "A" {
-
-  // factory constructors allow us to do computation ahead of the allocation.
-  factory A(int len) { return _construct(len); }
-
-  factory A.fromString(String s) {
-    return _construct(s.length);  // convert string to int.
-  }
-
-  // Helper that does the actual allocation and construction.
-  static A _construct(v) native @'return new A(v);';
-
-  foo() native 'return this._x;';
-}
-
-main() {
-  var a1 = new A(100);
-  var a2 = new A.fromString('Hello');
-
-  Expect.equals(100, a1.foo());
-  Expect.equals(5, a2.foo());
-}