Use more semantic exit codes so we can detect HTTP errors.

utils/pub/hosted_source.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.
 
 #library('hosted_source');
 
 #import('dart:json');
 #import('dart:uri');
 #import('io.dart');
 #import('package.dart');
@@ skipping 21 matching lines @@
    * site.
    */
   Future<List<Version>> getVersions(String name, description) {
     var parsed = _parseDescription(description);
     var fullUrl = "${parsed.last}/packages/${parsed.first}.json";
 
     return httpGetString(fullUrl).transform((body) {
       var doc = JSON.parse(body);
       return doc['versions'].map((version) => new Version.parse(version));
     }).transformException((ex) {
-      if (ex is HttpException && ex.statusCode == 404) {
+      if (ex is PubHttpException && ex.statusCode == 404) {
         throw 'Could not find package "${parsed.first}" on ${parsed.last}.';
       }
 
       // Otherwise re-throw the original exception.
       throw ex;
     });
   }
 
   /**
    * Downloads and parses the pubspec for a specific version of a package that
@@ skipping 77 matching lines @@
 
     var name = description["name"];
     if (name is! String) {
       throw new FormatException("The 'name' key must have a string value.");
     }
 
     var url = description.containsKey("url") ? description["url"] : defaultUrl;
     return new Pair<String, String>(name, url);
   }
 }