Migrate test block 8 to Dart 2.0.

tests/corelib/exception_implementation_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.
-
-library exception_implementation_test;
-
-import "package:expect/expect.dart";
-
-main() {
-  final msg = 1;
-  try {
-    throw new Exception(msg);
-    Expect.fail("Unreachable");
-  } on Exception catch (e) {
-    Expect.isTrue(e is Exception);
-    Expect.equals("Exception: $msg", e.toString());
-  }
-}