A type should be a subtype of Function if its class declares a call function,

tests/language/regress_25550_test.dart

 // Copyright (c) 2016, 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.
 
-// VMOptions=--no-support_ast_printer
+typedef int Adder(int a, int b);
+
+class Mock {
+  noSuchMethod(i) => null;
+}
+
+class MockAdder extends Mock {
+  int call(int a, int b);
+}
 
 main() {
-  print("Okay");
+  Adder adder = new MockAdder();
 }