Migrate pub away from throwing strings.

utils/pub/source_registry.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 source_registry;
 
 import 'source.dart';
 
 /// A class that keeps track of [Source]s used for installing packages.
 class SourceRegistry {
   final Map<String, Source> _map;
   Source _default;
 
   /// Creates a new registry with no packages registered.
   SourceRegistry() : _map = <String, Source>{};
 
   /// Returns the default source, which is used when no source is specified.
   Source get defaultSource => _default;
 
   /// Sets the default source. This takes a string, which must be the name of a
   /// registered source.
   void setDefault(String name) {
     if (!_map.containsKey(name)) {
-      // TODO(nweiz): Real error-handling system
-      throw 'Default source $name is not in the registry';
+      throw new StateError('Default source $name is not in the registry');
     }
 
     _default = _map[name];
   }
 
   /// Registers a new source. This source may not have the same name as a source
   /// that's already been registered.
   void register(Source source) {
     if (_map.containsKey(source.name)) {
-      // TODO(nweiz): Real error-handling system
-      throw 'Source registry already has a source named ${source.name}';
+      throw new StateError('Source registry already has a source named '
+          '${source.name}');
     }
 
     _map[source.name] = source;
   }
 
   /// Returns `true` if there is a source named [name].
   bool contains(String name) => _map.containsKey(name);
 
   /// Returns the source named [name]. Throws an error if no such source has
   /// been registered. If [name] is null, returns the default source.
   Source operator[](String name) {
     if (name == null) {
       if (defaultSource != null) return defaultSource;
-      // TODO(nweiz): Real error-handling system
-      throw 'No default source has been registered';
+      throw new StateError('No default source has been registered');
     }
     if (_map.containsKey(name)) return _map[name];
-    throw 'No source named $name is registered';
+    throw new ArgumentError('No source named $name is registered');
   }
 }