Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/closure.dart.strong.expect

Index: pkg/front_end/testcases/closure.dart.strong.expect
diff --git a/pkg/front_end/testcases/closure.dart.strong.expect b/pkg/front_end/testcases/closure.dart.strong.expect
index 01ba4e5a76646fbb1764947575e10ebfd432b55d..465bcdcde5b2ac0da3a267abeef5ba62f905e0cb 100644
--- a/pkg/front_end/testcases/closure.dart.strong.expect
+++ b/pkg/front_end/testcases/closure.dart.strong.expect
@@ -4,12 +4,12 @@ import "dart:core" as core;
 
 class Foo extends core::Object {
   field self::Bar _field = new self::Bar::•();
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Bar extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }