Add a Pub source that checks packages out from Git.

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.
+
+/**
+ * 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 = <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';
+    }
+
+    _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}';
+    }
+
+    _map[source.name] = source;
+  }
+
+  /**
+   * 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';
+    }
+    if (_map.containsKey(name)) return _map[name];
+    throw 'No source named $name is registered';
+  }
+}