Change allowInterop to a generic method.

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Support for interoperating with JavaScript.
  *
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 513 matching lines @@
 /// using the package:js Dart-JavaScript interop.
 ///
 /// For performance reasons in Dart2Js, by default Dart functions cannot be
 /// passed directly to JavaScript unless this method is called to create
 /// a Function compatible with both Dart and JavaScript.
 /// Calling this method repeatedly on a function will return the same function.
 /// The [Function] returned by this method can be used from both Dart and
 /// 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>();
 
 /// Returns a [Function] that when called from JavaScript captures its 'this'
 /// binding and calls [f] with the value of this passed as the first argument.
 /// When called from Dart, [null] will be passed as the first argument.
 ///
 /// See the documention for [allowInterop]. This method should only be used with
 /// package:js Dart-JavaScript interop.
 Function allowInteropCaptureThis(Function f) {
   var ret = _interopCaptureThisExpando[f];
   if (ret == null) {
     ret = JS('',
         'function(/*...arguments*/) {'
         '  let args = [this];'
         '  for (let arg of arguments) {'
         '    args.push(arg);'
         '  }'
         '  return #(...args);'
         '}',
         f);
     _interopCaptureThisExpando[f] = ret;
   }
   return ret;
 }