Change allowInterop to a generic method.

tool/input_sdk/lib/js/dart2js/js_dart2js.dart

Index: tool/input_sdk/lib/js/dart2js/js_dart2js.dart
diff --git a/tool/input_sdk/lib/js/dart2js/js_dart2js.dart b/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
index 74f1f6304e7e0f9cbed08626cfb016fd58e258d2..4e2d6fd9c6ce1f56ea61d7bad5fd9b97bf71297d 100644
--- a/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
+++ b/tool/input_sdk/lib/js/dart2js/js_dart2js.dart
@@ -531,7 +531,7 @@ Object _putIfAbsent(weakMap, o, getValue(o)) {
 /// JavaScript. We may remove the need to call this method completely in the
 /// future if Dart2Js is refactored so that its function calling conventions
 /// are more compatible with JavaScript.
-Function allowInterop(Function f) => f;
+Function /*=F*/ allowInterop/*<F extends Function>*/(Function /*=F*/ f) => f;
 
 Expando<Function> _interopCaptureThisExpando = new Expando<Function>();