Handle call properties in is-checks.

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index d4df75d16bc34ec30a8e6f3ed970bbb8516cebf2..e641095dd79499dc6c737d4257dd62498594f27c 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -1694,12 +1694,12 @@ class CodeEmitterTask extends CompilerTask {
     // [Function] and needs checks for all typedefs that are used in is-checks.
     if (checkedClasses.contains(compiler.functionClass) ||
         !checkedTypedefs.isEmpty) {
-      FunctionElement call = cls.lookupLocalMember(Compiler.CALL_OPERATOR_NAME);
+      Element call = cls.lookupLocalMember(Compiler.CALL_OPERATOR_NAME);
       if (call == null) {
         // If [cls] is a closure, it has a synthetic call operator method.
         call = cls.lookupBackendMember(Compiler.CALL_OPERATOR_NAME);
       }
-      if (call != null) {
+      if (call != null && call.isFunction()) {
         generateInterfacesIsTests(compiler.functionClass,
                                   emitIsTest,
                                   emitSubstitution,