Update charted to 0.4.8 and roll

packages/collection/lib/src/combined_wrappers/combined_map.dart

+// Copyright (c) 2017, 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 'combined_iterable.dart';
+
+/// Returns a new map that represents maps flattened into a single map.
+///
+/// All methods and accessors treat the new map as-if it were a single
+/// concatenated map, but the underlying implementation is based on lazily
+/// accessing individual map instances. In the occasion where a key occurs in
+/// multiple maps the first value is returned.
+///
+/// The resulting map has an index operator (`[]`) and `length` property that
+/// are both `O(maps)`, rather than `O(1)`, and the map is unmodifiable - but
+/// underlying changes to these maps are still accessible from the resulting
+/// map.
+class CombinedMapView<K, V> extends UnmodifiableMapBase<K, V> {
+  final Iterable<Map<K, V>> _maps;
+
+  /// Create a new combined view into multiple maps.
+  ///
+  /// The iterable is accessed lazily so it should be collection type like
+  /// [List] or [Set] rather than a lazy iterable produced by `map()` et al.
+  CombinedMapView(this._maps);
+
+  V operator [](Object key) {
+    for (var map in _maps) {
+      // Avoid two hash lookups on a positive hit.
+      var value = map[key];
+      if (value != null || map.containsKey(value)) {
+        return value;
+      }
+    }
+    return null;
+  }
+
+  /// The keys of [this].
+  ///
+  /// The returned iterable has efficient `length` and `contains` operations,
+  /// based on [length] and [containsKey] of the individual maps.
+  ///
+  /// The order of iteration is defined by the individual `Map` implementations,
+  /// but must be consistent between changes to the maps.
+  ///
+  /// Unlike most [Map] implementations, modifying an individual map while
+  /// iterating the keys will _sometimes_ throw. This behavior may change in
+  /// the future.
+  Iterable<K> get keys => new CombinedIterableView<K>(_maps.map((m) => m.keys));
+}