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

pkg/front_end/testcases/inference/parameter_defaults_downwards.dart.strong.expect

Index: pkg/front_end/testcases/inference/parameter_defaults_downwards.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/parameter_defaults_downwards.dart.strong.expect b/pkg/front_end/testcases/inference/parameter_defaults_downwards.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..72a4648b482462713373d1f40173ea9cbfdac059
--- /dev/null
+++ b/pkg/front_end/testcases/inference/parameter_defaults_downwards.dart.strong.expect
@@ -0,0 +1,12 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method optional_toplevel([core::List<core::int> x = const <core::int>[]]) → void {}
+static method named_toplevel({core::List<core::int> x = const <core::int>[]}) → void {}
+static method main() → dynamic {
+  function optional_local([core::List<core::int> x = const <core::int>[]]) → void {}
+  function named_local({core::List<core::int> x = const <core::int>[]}) → void {}
+  ([core::List<core::int>]) → core::Null optional_closure = ([core::List<core::int> x = const <core::int>[]]) → core::Null {};
+  ({x: core::List<core::int>}) → core::Null name_closure = ({core::List<core::int> x = const <core::int>[]}) → core::Null {};
+}