Provide and respect HTTP cache headers in pub serve.

lib/src/utils.dart

 // Copyright (c) 2012, 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.
 
 /// Generic utility functions. Stuff that should possibly be in core.
 import 'dart:async';
 import "dart:convert";
 import 'dart:io';
 
 import "package:crypto/crypto.dart";
@@ skipping 503 matching lines @@
   }, onError: (error, [stackTrace]) {
     // If the error came after values, it's OK.
     if (completer.isCompleted) {
       controller.addError(error, stackTrace);
       return;
     }
 
     // Otherwise, the error came first and the stream is invalid.
     completer.completeError(error, stackTrace);
 
-    // We don't be returning the stream at all in this case, so unsubscribe
+    // We won't be returning the stream at all in this case, so unsubscribe
     // and swallow the error.
     subscription.cancel();
   }, onDone: () {
     // It closed with no errors, so the stream is valid.
     if (!completer.isCompleted) completer.complete(controller.stream);
     controller.close();
   });
 
   return completer.future;
 }
@@ skipping 405 matching lines @@
   } else {
     throw new ApplicationException(message);
   }
 }
 
 /// Throw a [DataException] with [message] to indicate that the command has
 /// failed because of invalid input data.
 ///
 /// This will report the error and cause pub to exit with [exit_codes.DATA].
 void dataError(String message) => throw new DataException(message);