Fix greatest/least closure computation when there are multiple `?`s.

pkg/front_end/testcases/inference/greatest_closure_multiple_params.dart

Index: pkg/front_end/testcases/inference/greatest_closure_multiple_params.dart
diff --git a/pkg/front_end/testcases/inference/greatest_closure_multiple_params.dart b/pkg/front_end/testcases/inference/greatest_closure_multiple_params.dart
new file mode 100644
index 0000000000000000000000000000000000000000..687fbf737720290660c60b46e319d7c675d0bb6d
--- /dev/null
+++ b/pkg/front_end/testcases/inference/greatest_closure_multiple_params.dart
@@ -0,0 +1,22 @@
+// Copyright (c) 2017, 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.
+
+/*@testedFeatures=inference*/
+library test;
+
+abstract class C<E> {
+  void sort([int compare(E a, E b)]) {
+    /*@typeArgs=C::E*/ sort2(this, compare ?? _compareAny);
+  }
+
+  static int _compareAny(a, b) {
+    throw 'unimplemented';
+  }
+
+  static void sort2<E>(C<E> a, int compare(E a, E b)) {
+    throw 'unimplemented';
+  }
+}
+
+main() {}