Attach type info to extension method and object method tear-offs

lib/runtime/_classes.js

Index: lib/runtime/_classes.js
diff --git a/lib/runtime/_classes.js b/lib/runtime/_classes.js
index 8d897240653dac711a8847da835c3f239b5ae748..80ceb56249bf4fa36dca9b9729e255d701391dfd 100644
--- a/lib/runtime/_classes.js
+++ b/lib/runtime/_classes.js
@@ -336,6 +336,9 @@ dart_library.library('dart_runtime/_classes', null, /* Imports */[
       copyTheseProperties(jsProto, extProto, getOwnPropertySymbols(extProto));
       extProto = extProto.__proto__;
     }
+    let originalSigFn = getOwnPropertyDescriptor(dartExtType, _methodSig).get;
+    assert(originalSigFn);
+    defineMemoizedGetter(jsType, _methodSig, originalSigFn);
   }
   exports.registerExtension = registerExtension;