pkg/shelf: rename Stack to Pipeline

pkg/shelf/lib/src/pipeline.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.
 
-library shelf.stack;
+library shelf.pipeline;
 
 import 'handler.dart';
 import 'middleware.dart';
 
 /// A helper that makes it easy to compose a set of [Middleware] and a
 /// [Handler].
 ///
-///     var handler = const Stack()
+///     var handler = const Pipeline()
 ///         .addMiddleware(loggingMiddleware)
 ///         .addMiddleware(cachingMiddleware)
 ///         .addHandler(application);
-class Stack {
-  final Stack _parent;
+class Pipeline {
+  final Pipeline _parent;
   final Middleware _middleware;
 
-  const Stack()
+  const Pipeline()
       : _middleware = null,
         _parent = null;
 
-  Stack._(this._middleware, this._parent);
+  Pipeline._(this._middleware, this._parent);
 
-  /// Returns a new [Stack] with [middleware] added to the existing set of
+  /// Returns a new [Pipeline] with [middleware] added to the existing set of
   /// [Middleware].
   ///
   /// [middleware] will be the last [Middleware] to process a request and
   /// the first to process a response.
-  Stack addMiddleware(Middleware middleware) =>
-      new Stack._(middleware, this);
+  Pipeline addMiddleware(Middleware middleware) =>
+      new Pipeline._(middleware, this);
 
   /// Returns a new [Handler] with [handler] as the final processor of a
-  /// [Request] if all of the middleware in the stack have passed the request
+  /// [Request] if all of the middleware in the pipeline have passed the request
   /// through.
   Handler addHandler(Handler handler) {
     if (_middleware == null) return handler;
     return _parent.addHandler(_middleware(handler));
   }
 
-  /// Exposes this stack of [Middleware] as a single middleware instance.
+  /// Exposes this pipeline of [Middleware] as a single middleware instance.
   Middleware get middleware => addHandler;
 }