Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/argument.dart.strong.expect
diff --git a/pkg/front_end/testcases/argument.dart.strong.expect b/pkg/front_end/testcases/argument.dart.strong.expect
index 27c776c0b47008735d25cd7736deaa2fa478777f..bade0cb28e6641aedf1b9629f066f7da13049bc7 100644
--- a/pkg/front_end/testcases/argument.dart.strong.expect
+++ b/pkg/front_end/testcases/argument.dart.strong.expect
@@ -3,22 +3,22 @@ import self as self;
 import "dart:core" as core;
 
 abstract class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Foo extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
 }
 class Bar extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
 }
 class Baz extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
 }