Make Zone API strong mode clean.

tests/lib_strong/mirrors/invocation_fuzz_test.dart

 // Copyright (c) 2014, 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.
 
 // This test reflectively enumerates all the methods in the system and tries to
 // invoke them with various basic values (nulls, ints, etc). This may result in
 // Dart exceptions or hangs, but should never result in crashes or JavaScript
 // exceptions.
 
 library test.invoke_natives;
@@ skipping 172 matching lines @@
     0xFFFFFFFFFFFFFFFF,
     3.14159,
     "foo",
     'blåbærgrød',
     'Îñţérñåţîöñåļîžåţîờñ',
     "\u{1D11E}",
     #symbol
   ];
   valueObjects.forEach((v) => checkInstance(reflect(v), 'value object'));
 
-  uncaughtErrorHandler(self, parent, zone, error, stack) {}
-  ;
+  void uncaughtErrorHandler(self, parent, zone, error, stack) {}
+
   var zoneSpec =
       new ZoneSpecification(handleUncaughtError: uncaughtErrorHandler);
   testZone = Zone.current.fork(specification: zoneSpec);
   testZone.createTimer(Duration.ZERO, doOneTask);
 }