fixes #216, JSNumber members

lib/runtime/dart_runtime.js

Index: lib/runtime/dart_runtime.js
diff --git a/lib/runtime/dart_runtime.js b/lib/runtime/dart_runtime.js
index 3cb2c617ca74a889de05701803c4b73659ee0b13..1b6125ee36822a219a4bb7c4581b004dfb6289c2 100644
--- a/lib/runtime/dart_runtime.js
+++ b/lib/runtime/dart_runtime.js
@@ -799,7 +799,12 @@ var dart, dartx;
     // Mark the JS type's instances so we can easily check for extensions.
     assert(jsProto[_extensionType] === void 0);
     jsProto[_extensionType] = extProto;
-    copyPropertiesHelper(jsProto, extProto, getOwnPropertySymbols(extProto));
+
+    let dartObjProto = core.Object.prototype;
+    while (extProto !== dartObjProto && extProto !== jsProto) {
+      copyPropertiesHelper(jsProto, extProto, getOwnPropertySymbols(extProto));
+      extProto = extProto.__proto__;
+    }
   }
   dart.registerExtension = registerExtension;