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

lib/compiler/implementation/lib/coreimpl_patch.dart

Index: lib/compiler/implementation/lib/coreimpl_patch.dart
diff --git a/lib/compiler/implementation/lib/coreimpl_patch.dart b/lib/compiler/implementation/lib/coreimpl_patch.dart
index d48cafe2a2e68d31c025f486d57a4788a957aaa8..ee13f7c44ce8b8c1e383f529b85552a67c824373 100644
--- a/lib/compiler/implementation/lib/coreimpl_patch.dart
+++ b/lib/compiler/implementation/lib/coreimpl_patch.dart
@@ -199,3 +199,36 @@ class _AllMatchesIterator implements Iterator<Match> {
     }
   }
 }
+
+// Patch for Expando implementation.
+// TODO(ager): Split out into expando_patch.dart and allow #source in
+// patch files?
+patch class ExpandoImplementation<T> {
+
+  patch T operator[](Object object) {
+    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
+    return (values === null) ? null : Primitives.getProperty(values, _getKey());
+  }
+
+  patch void operator[]=(Object object, T value) {
+    var values = Primitives.getProperty(object, _EXPANDO_PROPERTY_NAME);
+    if (values === null) {
+      values = new Object();
+      Primitives.setProperty(object, _EXPANDO_PROPERTY_NAME, values);
+    }
+    Primitives.setProperty(values, _getKey(), value);
+  }
+
+  String _getKey() {
+    String key = Primitives.getProperty(this, _KEY_PROPERTY_NAME);
+    if (key === null) {
+      key = "expando\$key\$${_keyCount++}";
+      Primitives.setProperty(this, _KEY_PROPERTY_NAME, key);
+    }
+    return key;
+  }
+
+  static final String _KEY_PROPERTY_NAME = 'expando\$key';
+  static final String _EXPANDO_PROPERTY_NAME = 'expando\$values';
+  static int _keyCount = 0;
+}