Update charted to 0.4.8 and roll

packages/collection/lib/src/empty_unmodifiable_set.dart

+// Copyright (c) 2016, 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.
+
+import 'dart:collection';
+
+import 'unmodifiable_wrappers.dart';
+
+// Unfortunately, we can't use UnmodifiableSetMixin here, since const classes
+// can't use mixins.
+/// An unmodifiable, empty set that can have a const constructor.
+class EmptyUnmodifiableSet<E> extends IterableBase<E>
+    implements UnmodifiableSetView<E> {
+  static T _throw<T>() {
+    throw new UnsupportedError("Cannot modify an unmodifiable Set");
+  }
+
+  Iterator<E> get iterator => new Iterable<E>.empty().iterator;
+  int get length => 0;
+
+  const EmptyUnmodifiableSet();
+
+  bool contains(Object element) => false;
+  bool containsAll(Iterable<Object> other) => other.isEmpty;
+  E lookup(Object element) => null;
+  Set<E> toSet() => new Set();
+  Set<E> union(Set<E> other) => new Set.from(other);
+  Set<E> intersection(Set<Object> other) => new Set();
+  Set<E> difference(Set<Object> other) => new Set();
+
+  bool add(E value) => _throw();
+  void addAll(Iterable<E> elements) => _throw();
+  void clear() => _throw();
+  bool remove(Object element) => _throw();
+  void removeAll(Iterable<Object> elements) => _throw();
+  void removeWhere(bool test(E element)) => _throw();
+  void retainWhere(bool test(E element)) => _throw();
+  void retainAll(Iterable<Object> elements) => _throw();
+}