Provide an Expando implementation for the VM.

runtime/lib/expando.dart

+// Copyright (c) 2012, 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.
+
+class _Expando<T> implements Expando<T> {
+  final String name;
+
+  const _Expando([String this.name]);
+
+  T operator[](Object object) {
+    checkType(object);
+    var weak_property = find(this);
+    var list = weak_property.value;
+    var do_compact = false;
+    var result = null;
+    for (int i = 0; i < list.length; ++i) {
+      var key = list[i].key;
+      if (key === object) {
+        result = list[i].value;
+        break;
+      }
+      if (key === null) {
+        do_compact = true;
+        list[i] = null;
+      }
+    }
+    if (do_compact) {
+      weak_property.value = list.filter((e) => (e !== null));
+    }
+    return result;
+  }
+
+  void operator[]=(Object object, T value) {
+    checkType(object);
+    var weak_property = find(this);
+    var list = weak_property.value;
+    var do_compact = false;
+    int i = 0;
+    for (; i < list.length; ++i) {
+      var key = list[i].key;
+      if (key === object) {
+        break;
+      }
+      if (key === null) {
+        do_compact = true;
+        list[i] = null;
+      }
+    }
+    if (i !== list.length && value === null) {
+      do_compact = true;
+      list[i] = null;
+    } else if (i !== list.length) {
+      list[i].value = value;
+    } else {
+      list.add(new _WeakProperty(object, value));
+    }
+    if (do_compact) {
+      weak_property.value = list.filter((e) => (e !== null));
+    }
+  }
+
+  String toString() => "Expando:$name";
+
+  static checkType(object) {
+    if (object === null) {
+      throw new NullPointerException();
+    }
+    if (object is bool || object is num || object is String) {
+      throw new IllegalArgumentException(object);
+    }
+  }
+
+  static find(expando) {
+    if (data === null) data = new List();
+    var do_compact = false;
+    int i = 0;
+    for (; i < data.length; ++i) {
+      var key = data[i].key;
+      if (key == expando) {
+        break;
+      }
+      if (key === null) {
+        do_compact = true;
+        data[i] = null;
+      }
+    }
+    if (i == data.length) {
+      data.add(new _WeakProperty(expando, new List()));
+    }
+    var result = data[i];
+    if (do_compact) {
+      data = data.filter((e) => (e !== null));
+    }
+    return result;
+  }
+
+  static List data;
+}