Roll to pickup pool changes

packages/pool/README.md

 The pool package exposes a `Pool` class which makes it easy to manage a limited
 pool of resources.
 
 The easiest way to use a pool is by calling `withResource`. This runs a callback
 and returns its result, but only once there aren't too many other callbacks
 currently running.
 
 ```dart
 // Create a Pool that will only allocate 10 resources at once. After 30 seconds
 // of inactivity with all resources checked out, the pool will throw an error.
 final pool = new Pool(10, timeout: new Duration(seconds: 30));
 
 Future<String> readFile(String path) {
   // Since the call to [File.readAsString] is within [withResource], no more
   // than ten files will be open at once.
-  return pool.withResource(() => return new File(path).readAsString());
+  return pool.withResource(() => new File(path).readAsString());
 }
 ```
 
 For more fine-grained control, the user can also explicitly request generic
 `PoolResource` objects that can later be released back into the pool. This is
 what `withResource` does under the covers: requests a resource, then releases it
 once the callback completes.
 
 `Pool` ensures that only a limited number of resources are allocated at once.
 It's the caller's responsibility to ensure that the corresponding physical
@@ skipping 17 matching lines @@
   // ...
 
   Future<RandomAccessFile> close() {
     return _file.close.then((_) {
       _resource.release();
       return this;
     });
   }
 }
 ```