Provide and respect HTTP cache headers in pub serve.

lib/src/command/serve.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.
 
 import 'dart:async';
 import 'dart:math' as math;
 
 import 'package:barback/barback.dart';
 
 import '../barback/asset_environment.dart';
@@ skipping 132 matching lines @@
     // been compiled to JavaScript already.
     if (mode == BarbackMode.RELEASE) {
       server.allowAsset = (url) => !url.path.endsWith(".dart");
     }
 
     // Add two characters to account for "[" and "]".
     var directory = log.gray(
         padRight("[${server.rootDirectory}]", directoryLength + 2));
 
     server.results.listen((result) {
-      if (result.isSuccess) {
+      if (result.isCached) {
+        var prefix = "$directory ${log.green('GET')}";
+        log.collapsible(
+            "$prefix ${result.url.path} $_arrow (cached) ${result.id}",
+            "$prefix Served ## cached assets.");
+      } else if (result.isSuccess) {
         var prefix = "$directory ${log.green('GET')}";
         log.collapsible("$prefix ${result.url.path} $_arrow ${result.id}",
             "$prefix Served ## assets.");
       } else {
         var buffer = new StringBuffer();
         buffer.write("$directory ${log.red('GET')} ${result.url.path} $_arrow");
 
         var error = result.error.toString();
         if (error.contains("\n")) {
           buffer.write("\n${prefixLines(error)}");
         } else {
           buffer.write(" $error");
         }
 
         log.error(buffer);
       }
     }, onError: _fatalError);
 
     log.message("Serving ${entrypoint.root.name} "
         "${padRight(server.rootDirectory, directoryLength)} "
         "on ${log.bold('http://$hostname:${server.port}')}");
   }
 
   /// Reports [error] and exits the server.
   void _fatalError(error, [stackTrace]) {
     if (_completer.isCompleted) return;
     _completer.completeError(error, stackTrace);
   }
 }