Reverting 7623

utils/pub/source.dart

Index: utils/pub/source.dart
diff --git a/utils/pub/source.dart b/utils/pub/source.dart
index da01f1214930d47b5c335afc66334e182f770407..3e4eb087c0be0f89a957844e13fef924311c8778 100644
--- a/utils/pub/source.dart
+++ b/utils/pub/source.dart
@@ -10,6 +10,22 @@
  */
 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.
    */