Fixed strong mode errors and warnings reachable from lib/observe.dart

lib/src/dirty_check.dart

 // Copyright (c) 2013, 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.
 
 /// *Warning*: this library is **internal**, and APIs are subject to change.
 ///
 /// Tracks observable objects for dirty checking and testing purposes.
 ///
 /// It can collect all observed objects, which can be used to trigger
 /// predictable delivery of all pending changes in a test, including objects
 /// allocated internally to another library, such as those in
 /// `package:template_binding`.
 library observe.src.dirty_check;
 
 import 'dart:async';
+import 'package:func/func.dart';
 import 'package:logging/logging.dart';
 import 'package:observe/observe.dart' show Observable;
 
 /// The number of active observables in the system.
 int get allObservablesCount => _allObservablesCount;
 
 int _allObservablesCount = 0;
 
 List<Observable> _allObservables = null;
 
@@ skipping 70 matching lines @@
     // Only schedule one dirty check per microtask.
     if (pending) return;
 
     pending = true;
     parent.scheduleMicrotask(zone, () {
       pending = false;
       Observable.dirtyCheck();
     });
   }
 
-  wrapCallback(Zone self, ZoneDelegate parent, Zone zone, f()) {
+  Func0 wrapCallback(Zone self, ZoneDelegate parent, Zone zone, f()) {
     // TODO(jmesserly): why does this happen?
     if (f == null) return f;
     return () {
       enqueueDirtyCheck(parent, zone);
       return f();
     };
   }
 
-  wrapUnaryCallback(Zone self, ZoneDelegate parent, Zone zone, f(x)) {
+  Func1 wrapUnaryCallback(Zone self, ZoneDelegate parent, Zone zone, f(x)) {
     // TODO(jmesserly): why does this happen?
     if (f == null) return f;
     return (x) {
       enqueueDirtyCheck(parent, zone);
       return f(x);
     };
   }
 
   return new ZoneSpecification(
-      registerCallback: wrapCallback,
-      registerUnaryCallback: wrapUnaryCallback);
+      registerCallback: wrapCallback, registerUnaryCallback: wrapUnaryCallback);
 }
 
 /// Forks a [Zone] off the current one that does dirty-checking automatically
 /// after each batch of async operations. Equivalent to:
 ///
 ///     Zone.current.fork(specification: dirtyCheckZoneSpec());
 Zone dirtyCheckZone() => Zone.current.fork(specification: dirtyCheckZoneSpec());