Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/super_rasta_copy.dart.outline.expect
diff --git a/pkg/front_end/testcases/super_rasta_copy.dart.outline.expect b/pkg/front_end/testcases/super_rasta_copy.dart.outline.expect
index 91d4f2a977c9c432f2f1d5eddbb11b8764611c7f..61dc3881eba3d46ecbcdd8d5ac27ba8fdfc22ecc 100644
--- a/pkg/front_end/testcases/super_rasta_copy.dart.outline.expect
+++ b/pkg/front_end/testcases/super_rasta_copy.dart.outline.expect
@@ -8,7 +8,7 @@ class A extends core::Object {
   field dynamic c;
   field dynamic d;
   final field dynamic f;
-  constructor •() → void
+  default constructor •() → void
     ;
   get e() → dynamic
     ;
@@ -35,7 +35,7 @@ class A extends core::Object {
 }
 class B extends self::A {
   final field dynamic d;
-  constructor •() → void
+  default constructor •() → void
     ;
   get b() → dynamic
     ;
@@ -45,7 +45,7 @@ class B extends self::A {
     ;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     ;
   method test() → dynamic
     ;