Update charted to 0.4.8 and roll

packages/petitparser/lib/src/lisp/environment.dart

-part of petitparser.lisp;
-
-/// Environment of bindings.
-class Environment {
-
-  /// The owning environment.
-  final Environment _owner;
-
-  /// The internal environment bindings.
-  final Map<Name, dynamic> _bindings;
-
-  /// Constructor for the nested environment.
-  Environment([this._owner]) : _bindings = new Map();
-
-  /// Constructor for a nested environment.
-  Environment create() => new Environment(this);
-
-  /// Return the binding for [key].
-  operator [](Name key) {
-    if (_bindings.containsKey(key)) {
-      return _bindings[key];
-    } else if (_owner != null) {
-      return _owner[key];
-    } else {
-      return _invalidBinding(key);
-    }
-  }
-
-  /// Updates the binding for [key] with a [value].
-  void operator []=(Name key, value) {
-    if (_bindings.containsKey(key)) {
-      _bindings[key] = value;
-    } else if (_owner != null) {
-      _owner[key] = value;
-    } else {
-      _invalidBinding(key);
-    }
-  }
-
-  /// Defines a new binding from [key] to [value].
-  define(Name key, value) {
-    return _bindings[key] = value;
-  }
-
-  /// Returns the keys of the bindings.
-  Iterable<Name> get keys => _bindings.keys;
-
-  /// Returns the parent of the bindings.
-  Environment get owner => _owner;
-
-  /// Called when a missing binding is accessed.
-  _invalidBinding(Name key) {
-    throw new ArgumentError('Unknown binding for $key');
-  }
-}