Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/override.dart.direct.expect
diff --git a/pkg/front_end/testcases/override.dart.direct.expect b/pkg/front_end/testcases/override.dart.direct.expect
index c46b18731a3fa6c91377127868a5d49ec616c2c5..5448dbe16522d94fddef8cc834c1fd42d7e0ccbd 100644
--- a/pkg/front_end/testcases/override.dart.direct.expect
+++ b/pkg/front_end/testcases/override.dart.direct.expect
@@ -3,17 +3,17 @@ import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Bar extends self::Foo {
-  constructor •() → void
+  default constructor •() → void
     : super self::Foo::•()
     ;
 }
 class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method method() → self::Foo {
@@ -21,7 +21,7 @@ class Base extends core::Object {
   }
 }
 class Sub extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
   method method() → self::Foo {