Use WeakMap to support Expando if available.

runtime/lib/expando_patch.dart

Index: runtime/lib/expando_patch.dart
diff --git a/runtime/lib/expando_patch.dart b/runtime/lib/expando_patch.dart
index 0c9427ca41c7ad1f83710b8ee453c6932f8f5c7f..df0234d4d8ab0fb01a513ea64094f20852b38e90 100644
--- a/runtime/lib/expando_patch.dart
+++ b/runtime/lib/expando_patch.dart
@@ -131,7 +131,8 @@ patch class Expando<T> {
          (object is bool) ||
          (object is num) ||
          (object is String)) {
-      throw new ArgumentError(object);
+      throw new ArgumentError.value(object,
+          "Expandos are not allowed on strings, numbers, booleans or null");
     }
   }