Migrate block 163 to Dart 2.0.

tests/language_2/unhandled_exception_negative_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.
 // Dart test program for testing unhandled exceptions.
 
 class MyException {
-  const MyException(String message = "") : message_ = message;
+  const MyException(String message) : message_ = message;
   String getMessage() { return message_; }
   final String message_;
 }
 
 class Helper {
   static int f1(int i) {
     int j;
     j = i + 200;
     j = j + 300;
     throw new MyException("Unhandled Exception");
     return i;
   }
 }
 
 class UnhandledExceptionNegativeTest {
   static testMain() {
     Helper.f1(1);
   }
 }
 
 main() {
   UnhandledExceptionNegativeTest.testMain();
 }