Use patching for Expando so the implementation-specific files are not needed in the SDK.

runtime/lib/expando_impl.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 weakProperty = find(this);
-    var list = weakProperty.value;
-    var doCompact = 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) {
-        doCompact = true;
-        list[i] = null;
-      }
-    }
-    if (doCompact) {
-      weakProperty.value = list.filter((e) => (e !== null));
-    }
-    return result;
-  }
-
-  void operator[]=(Object object, T value) {
-    checkType(object);
-    var weakProperty = find(this);
-    var list = weakProperty.value;
-    var doCompact = false;
-    int i = 0;
-    for (; i < list.length; ++i) {
-      var key = list[i].key;
-      if (key === object) {
-        break;
-      }
-      if (key === null) {
-        doCompact = true;
-        list[i] = null;
-      }
-    }
-    if (i !== list.length && value === null) {
-      doCompact = true;
-      list[i] = null;
-    } else if (i !== list.length) {
-      list[i].value = value;
-    } else {
-      list.add(new _WeakProperty(object, value));
-    }
-    if (doCompact) {
-      weakProperty.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 doCompact = false;
-    int i = 0;
-    for (; i < data.length; ++i) {
-      var key = data[i].key;
-      if (key == expando) {
-        break;
-      }
-      if (key === null) {
-        doCompact = true;
-        data[i] = null;
-      }
-    }
-    if (i == data.length) {
-      data.add(new _WeakProperty(expando, new List()));
-    }
-    var result = data[i];
-    if (doCompact) {
-      data = data.filter((e) => (e !== null));
-    }
-    return result;
-  }
-
-  static List data;
-}