Remove old frog-style dummy bodies from dart2js native class tests

tests/compiler/dart2js_native/native_exceptions1_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 "dart:_js_helper";
 import "package:expect/expect.dart";
 
 // Test that hidden native exception classes can be marked as existing.
 //
 // To infer which native hidden types exist, we need
@@ skipping 10 matching lines @@
 @Native("E")
 class E {
   E._used() native;  // Bogus native constructor, called only from fake body.
 
   final int code;
 }
 
 // Type with exception-throwing methods.
 @Native("A")
 class A {
-  op(int x) native {
-    // Fake body calls constructor to mark the exception class (E) as used.
-    throw new E._used();
-  }
+  // Exception class E is created.
+  @Creates("E")
+  @Returns('int')
+  op(int x) native;
 }
 
 // This class is here just so that a dynamic context is polymorphic.
 class B {
   int get code => 666;
   op(String x) => 123;
 }
 
 makeA() native;
 
@@ skipping 50 matching lines @@
   threw = false;
   try {
     var x = aa.op(51);
   } on E catch (e) {
     threw = true;
     Expect.equals(100, e.code);
     Expect.isTrue(e is E);
   }
   Expect.isTrue(threw);
 }