Make is checks work on JavaScriptFunction objects.

pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart

Index: pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
index ae4cbba9da9adb9d8a8d795c138889cd49dfc683..ddef40bd3004ae03c2ee89196530571ad7c70c1b 100644
--- a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
@@ -132,6 +132,17 @@ class RuntimeTypeGenerator {
     _generateIsTestsOn(classElement, generateIsTest,
         generateFunctionTypeSignature, generateSubstitution, generateTypeCheck);
 
+    if (classElement == backend.helpers.jsJavaScriptFunctionClass) {
+        var type = backend.jsInteropAnalysis.buildJsFunctionType();
+        if (type != null) {
+          jsAst.Expression thisAccess = new jsAst.This();
+          RuntimeTypesEncoder rtiEncoder = backend.rtiEncoder;
+          jsAst.Expression encoding =
+              rtiEncoder.getSignatureEncoding(type, thisAccess);
+          jsAst.Name operatorSignature = namer.asName(namer.operatorSignature);
+          result.properties[operatorSignature] = encoding;
+        }
+    }
     return result;
   }