Roll Observatory packages and add a roll script

quiver/lib/testing/src/runtime/checked_mode.dart

-// Copyright 2013 Google Inc. All Rights Reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-part of quiver.testing.runtime;
-
-/**
- * Asserts that the current runtime has checked mode enabled.
- *
- * Otherwise, throws [StateError].
- */
-void assertCheckedMode() {
-  if (_isCheckedMode == null) _isCheckedMode = _checkForCheckedMode();
-
-  if (!_isCheckedMode) {
-    throw new StateError('Not in checked mode.');
-  }
-}
-
-bool _isCheckedMode = null;
-
-bool _checkForCheckedMode() {
-  Object sentinal = new Object();
-  try {
-    var i = 1 as dynamic;
-    String string = i;
-    throw sentinal;
-  } catch (e) {
-    if (e == sentinal) return false;
-  }
-  return true;
-}