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

lib/src/bind_property.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.
 
 library observe.src.bind_property;
 
 import 'dart:async';
 import 'package:observe/observe.dart';
 
 /// Forwards an observable property from one object to another. For example:
 ///
 ///     class MyModel extends Observable {
 ///       StreamSubscription _sub;
 ///       MyOtherModel _otherModel;
 ///
 ///       MyModel() {
 ///         ...
 ///         _sub = onPropertyChange(_otherModel, #value,
 ///             () => notifyPropertyChange(#prop, oldValue, newValue);
 ///       }
 ///
 ///       String get prop => _otherModel.value;
 ///       set prop(String value) { _otherModel.value = value; }
 ///     }
 ///
 /// See also [notifyPropertyChange].
 // TODO(jmesserly): make this an instance method?
-StreamSubscription onPropertyChange(Observable source, Symbol sourceName,
-    void callback()) {
+StreamSubscription onPropertyChange(
+    Observable source, Symbol sourceName, void callback()) {
   return source.changes.listen((records) {
     for (var record in records) {
       if (record is PropertyChangeRecord &&
           (record as PropertyChangeRecord).name == sourceName) {
         callback();
         break;
       }
     }
   });
 }