Tighten up handling of JS types and add test.

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.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.
 
 /// This library defines runtime operations on objects used by the code
 /// generator.
 part of dart._runtime;
 
 class InvocationImpl extends Invocation {
   final Symbol memberName;
@@ skipping 421 matching lines @@
         !!$isSubtype(actual, $StreamSubscription)) {
       console.warn('Ignoring cast fail from ' + $typeName(actual) +
                    ' to ' + $typeName(type));
       return true;
     }
     return false;
   });
 })()''');
 
 /// Returns true if [obj] is an instance of [type]
+/// Returns true if [obj] is a JS function and [type] is a function type
 /// Returns false if [obj] is not an instance of [type] in both spec
 ///  and strong mode
 /// Returns null if [obj] is not an instance of [type] in strong mode
 ///  but might be in spec mode
 bool strongInstanceOf(obj, type, ignoreFromWhiteList) => JS(
     '',
     '''(() => {
   let actual = $getReifiedType($obj);
   let result = $isSubtype(actual, $type);
-  if (result || actual == $jsobject ||
-      (actual == $int && $isSubtype($double, $type))) return true;
+  if (result || (actual == $int && $isSubtype($double, $type))) return true;
+  if (actual == $jsobject && $isFunctionType(type) && typeof(obj) === 'function') return true;

[vsm, 2017/04/21 12:35:58]

nit: line len

   if (result === false) return false;
   if (!$_ignoreWhitelistedErrors || ($ignoreFromWhiteList == void 0)) return result;
   if ($_ignoreTypeFailure(actual, $type)) return true;
   return result;
 })()''');
 
 /// Returns true if [obj] is null or an instance of [type]
 /// Returns false if [obj] is non-null and not an instance of [type]
 /// in strong mode
 instanceOfOrNull(obj, type) => JS(
@@ skipping 465 matching lines @@
     name = '+' + name;
   }
   return name;
 }
 
 /// Emulates the implicit "loadLibrary" function provided by a deferred library.
 ///
 /// Libraries are not actually deferred in DDC, so this just returns a future
 /// that completes immediately.
 Future loadLibrary() => new Future.value();