Update charted to 0.4.8 and roll

packages/quiver/lib/core.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.
 
-/**
- * Simple code with broad use cases.
- */
+/// Simple code with broad use cases.
 library quiver.core;
 
+import 'dart:collection';
+
 part 'src/core/hash.dart';
 part 'src/core/optional.dart';
 
-/**
- * Returns the first non-null argument. If all arguments are null, throws
- * an [ArgumentError].
- *
- * Note: if [o1] is an [Optional], this can be accomplished with `o1.or(o2)`.
- */
+/// Returns the first non-null argument. If all arguments are null, throws an
+/// [ArgumentError].
+///
+/// Note: if [o1] is an [Optional], this can be accomplished with `o1.or(o2)`.
 firstNonNull(o1, o2, [o3, o4]) {
   if (o1 != null) return o1;
   if (o2 != null) return o2;
   if (o3 != null) return o3;
   if (o4 != null) return o4;
   throw new ArgumentError('All arguments were null');
 }