Add a Pub source that checks packages out from Git.

utils/pub/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.
 
 /**
  * A source from which to install packages.
  *
  * Each source has many packages that it looks up using [PackageId]s. The source
  * is responsible for installing these packages to the package cache.
  */
 class Source {
   /**
-   * The default [Source] from which to fetch packages if no other [Source] is
-   * specified.
-   */
-  static Source defaultSource;
-
-  /**
-   * Looks up a source based on its name.
-   */
-  static Source fromName(String name) {
-    // TODO(nweiz): add a more principled way of registering sources here once
-    // we have more than one source. Especially important for plugins.
-    if (name == 'sdk') return defaultSource;
-    throw 'Unknown source "$name"';
-  }
-
-  /**
    * The name of the source. Should be lower-case, suitable for use in a
    * filename, and unique accross all sources.
    */
   abstract String get name();
 
   /**
    * Whether this source's packages should be cached in Pub's global cache
    * directory.
    *
    * A source should be cached if it requires network access to retrieve
@@ skipping 23 matching lines @@
    * resolution logic.
    *
    * This method should be light-weight. It doesn't need to validate that the
    * given package exists.
    *
    * The package name should be lower-case and suitable for use in a filename.
    * It may contain forward slashes.
    */
   String packageName(PackageId id) => id.fullName;
 }