Roll Observatory packages and add a roll script

analyzer/lib/src/util/asserts.dart

-// Copyright (c) 2015, 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 engine.utilities.asserts;
-
-/**
- * Ensures that the given [value] is not null.
- * Otherwise throws an [ArgumentError].
- * An optional [description] is used in the error message.
- */
-void notNull(Object value, [String description]) {
-  if (value == null) {
-    if (description == null) {
-      throw new ArgumentError('Must not be null');
-    } else {
-      throw new ArgumentError('Must not be null: $description');
-    }
-  }
-}