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 do_compact = false;
+    var result = null;
+    var list = weak_property.value;
+    for (int i = 0; i < list.length; ++i) {
+      if (list[i] !== null) {

[Ivan Posva, 2012/08/21 23:19:40]

How can this ever be false?

[cshapiro, 2012/08/22 02:37:35]

This was needed before I added compaction.  Since this invariant is now
maintained through the call to filter, I have removed the check here and in the
two similarly structured loops below.

+        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 do_compact = false;
+    var list = weak_property.value;
+    int i = 0;
+    for (; i < list.length; ++i) {
+      if (list[i] !== null) {
+        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 {
+      var weak_property = new WeakProperty(object, value);

[Ivan Posva, 2012/08/21 23:19:40]

It is confusing to be shadowing the outer weak_property here.

[cshapiro, 2012/08/22 02:37:35]

I removed this as part of an unrelated simplification.

+      if (i !== list.length) {
+        list[i] = weak_property;

[Ivan Posva, 2012/08/21 23:19:40]

Why not "list[i].value = value"?

[cshapiro, 2012/08/22 02:37:35]

While I added a setter for the value field, my preference is to have weak
properties be immutable objects.  Having said that, I use the assignment when I
filter out null elements and I may as well use it here as well.

+      } else {
+        list.add(weak_property);
+      }
+    }
+    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) {
+      if (data[i] !== null) {
+        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;
+}