Environment of bindings.

class Environment {

 /** The owning environemnt. */
 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]. */
 dynamic 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, dynamic 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]. */
 dynamic define(Name key, dynamic 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. */
 dynamic _invalidBinding(Name key) {
   throw new ArgumentError('Unknown binding for $key');
 }

}