Migrate pub away from throwing strings.

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:async';
 import 'dart:io' as io;
 import 'dart:json' as json;
 import 'dart:uri';
@@ skipping 115 matching lines @@
     return listDir(path.join(cacheDir)).map((entry) =>
         new Package.load(null, entry, systemCache.sources)).toList();
   }
   
   /// When an error occurs trying to read something about [package] from [url],
   /// this tries to translate into a more user friendly error message. Always
   /// throws an error, either the original one or a better one.
   void _throwFriendlyError(error, package, url) {
     if (error is PubHttpException &&
         error.response.statusCode == 404) {
-      throw 'Could not find package "$package" at $url.';
+      fail('Could not find package "$package" at $url.');
     }
 
     if (error is TimeoutException) {
-      throw 'Timed out trying to find package "$package" at $url.';
+      fail('Timed out trying to find package "$package" at $url.');
     }
 
     if (error is io.SocketIOException) {
-      throw 'Got socket error trying to find package "$package" at $url.\n'
-          '${error.osError}';
+      fail('Got socket error trying to find package "$package" at $url.\n'
+          '${error.osError}');
     }
 
     // Otherwise re-throw the original exception.
     throw error;
   }
 
 }
 
 /// The URL of the default package repository.
 final _defaultUrl = "https://pub.dartlang.org";
@@ skipping 44 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);
 }