Implement type variables on local function declarations and expressions.

pkg/front_end/testcases/illegal_named_function_expression.dart.direct.expect

+library;
+import self as self;
+import "dart:core" as core;
+
+const field dynamic #errors = const <dynamic>["pkg/front_end/testcases/illegal_named_function_expression.dart:6:11: Error: A function expression can't have a return type.\n  var x = void f<T>(T t) {};\n          ^", "pkg/front_end/testcases/illegal_named_function_expression.dart:6:16: Error: A function expression can't have a name.\n  var x = void f<T>(T t) {};\n               ^", "pkg/front_end/testcases/illegal_named_function_expression.dart:8:9: Error: A function expression can't have a return type.\n  print(void g<T>(T t) {});\n        ^", "pkg/front_end/testcases/illegal_named_function_expression.dart:8:14: Error: A function expression can't have a name.\n  print(void g<T>(T t) {});\n             ^"]/* from null */;
+static method main() → dynamic {
+  dynamic x = let final <T extends core::Object>(T) → void f = <T extends core::Object>(T t) → void {} in f;
+  core::print(x.runtimeType);
+  core::print(let final <T extends core::Object>(T) → void g = <T extends core::Object>(T t) → void {} in g);
+}