Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/argument.dart.outline.expect
diff --git a/pkg/front_end/testcases/argument.dart.outline.expect b/pkg/front_end/testcases/argument.dart.outline.expect
index b270ee0d1e467d6a4fab86f1b168a7f75829ec3c..525fe92783d8caf0ac0b896e4670011d7358a322 100644
--- a/pkg/front_end/testcases/argument.dart.outline.expect
+++ b/pkg/front_end/testcases/argument.dart.outline.expect
@@ -3,19 +3,19 @@ import self as self;
 import "dart:core" as core;
 
 abstract class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Foo extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Bar extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Baz extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method foo(dynamic x) → void