Tighten up handling of JS types and add test.

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

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
index 5299374ca4c7a8c8b2feb2136cd40eca1b1416ac..35f706be3f519ac02247376421c8a99dcc4d41b0 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
@@ -439,6 +439,7 @@ final _ignoreTypeFailure = JS(
 })()''');
 
 /// 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
@@ -448,8 +449,8 @@ 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;