Tighten up handling of JS types and add test.

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
index a0f02c7ac40553dacd76821115e80be28454f948..12d984b0cdfbfb10f7155174e9ab7be3112e081a 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -271,11 +271,16 @@ bind(obj, name, f) => JS(
     '',
     '''(() => {
   if ($f === void 0) $f = $obj[$name];
-  $f = $f.bind($obj);
   // TODO(jmesserly): track the function's signature on the function, instead
   // of having to go back to the class?
   let sig = $getMethodType($getType($obj), $name);
-  $assert_(sig);
+
+  // JS interop case: do not bind this for compatibility with the dart2js
+  // implementation where we cannot bind this reliably here until we trust
+  // types more.
+  if (sig === void 0) return $f;
+
+  $f = $f.bind($obj);
   $tag($f, sig);
   return $f;
 })()''');