Perform type inference on default values of optional/named parameters.

pkg/front_end/testcases/inference/parameter_defaults_downwards.dart

+// 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;
+
+void optional_toplevel([List<int> x = /*@typeArgs=int*/ const []]) {}
+
+void named_toplevel({List<int> x: /*@typeArgs=int*/ const []}) {}
+
+main() {
+  void optional_local([List<int> x = /*@typeArgs=int*/ const []]) {}
+  void named_local({List<int> x: /*@typeArgs=int*/ const []}) {}
+  var /*@type=([List<int>]) -> Null*/ optional_closure = /*@returnType=Null*/ (
+      [List<int> x = /*@typeArgs=int*/ const []]) {};
+  var /*@type=({x: List<int>}) -> Null*/ name_closure = /*@returnType=Null*/ (
+      {List<int> x: /*@typeArgs=int*/ const []}) {};
+}