Fix most strong mode warnings.

lib/src/async_memoizer.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:async';
 
 /// A class for running an asynchronous function exactly once and caching its
 /// result.
 ///
 /// An `AsyncMemoizer` is used when some function may be run multiple times in
@@ skipping 13 matching lines @@
 ///   Future close() => _closeMemo.runOnce(() {
 ///     // ...
 ///   });
 /// }
 /// ```
 class AsyncMemoizer<T> {
   /// The future containing the method's result.
   ///
   /// This can be accessed at any time, and will fire once [runOnce] is called.
   Future<T> get future => _completer.future;
-  final _completer = new Completer();
+  final _completer = new Completer<T>();
 
   /// Whether [runOnce] has been called yet.
   bool get hasRun => _completer.isCompleted;
 
   /// Runs the function, [computation], if it hasn't been run before.
   ///
   /// If [runOnce] has already been called, this returns the original result.
   Future<T> runOnce(computation()) {
     if (!hasRun) _completer.complete(new Future.sync(computation));
     return future;
   }
 }