Add a note to Source#install explaining that it should be very light-weight.

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.
 
 #library('source');
 
 #import('io.dart');
 #import('package.dart');
 #import('pubspec.dart');
 #import('system_cache.dart');
@@ skipping 85 matching lines @@
 
   /**
    * Installs the package identified by [id] to [path]. Returns a [Future] that
    * completes when the installation was finished. The [Future] should resolve
    * to true if the package was found in the source and false if it wasn't. For
    * all other error conditions, it should complete with an exception.
    *
    * [path] is guaranteed not to exist, and its parent directory is guaranteed
    * to exist.
    *
+   * Note that [path] may be deleted. If re-installing a package that has
+   * already been installed would be costly or impossible,
+   * [installToSystemCache] should be implemented instead of [install].
+   *
    * This doesn't need to be implemented if [installToSystemCache] is
    * implemented.
    */
   Future<bool> install(PackageId id, String path) {
     throw "Either install or installToSystemCache must be implemented for "
         "source $name.";
   }
 
   /**
    * Installs the package identified by [id] to the system cache. This is only
@@ skipping 73 matching lines @@
    *
    * The returned [PackageId] may have a description field that's invalid
    * according to [validateDescription], although it must still be serializable
    * to JSON and YAML. It must also be equal to [id] according to
    * [descriptionsEqual].
    *
    * By default, this just returns [id].
    */
   Future<PackageId> resolveId(PackageId id) => new Future.immediate(id);
 }