Fix dynamic calls on JS interop classes.

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 e91d790839ffbc3f9cb6bedcf6afedbd65988763..cdc14c1c37a9279066a7ac4831f04da8572ddd81 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
@@ -203,6 +203,21 @@ getType(obj) => JS(
   return $obj == null ? $Object : $obj.__proto__.constructor;
 })()''');
 
+bool isJsInterop(obj) {
+  if (JS('bool', 'typeof # === "function"', obj)) {
+    // A function is a Dart function if it has runtime type information.
+    return _getRuntimeType(obj) == null;
+  }
+  // Primitive types are not JS interop types.
+  if (JS('bool', 'typeof # !== "object"', obj)) return false;
+
+  // Extension types are not considered JS interop types.
+  // Note that it is still possible to call typed JS interop methods on
+  // extension types but the calls must be statically typed.
+  if (getExtensionType(obj) != null) return false;
+  return JS('bool', '!($obj instanceof $Object)');
+}
+
 /// Get the type of a method from a type using the stored signature
 getMethodType(type, name) => JS(
     '',