Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/generic_factory.dart.direct.expect

Index: pkg/front_end/testcases/rasta/generic_factory.dart.direct.expect
diff --git a/pkg/front_end/testcases/rasta/generic_factory.dart.direct.expect b/pkg/front_end/testcases/rasta/generic_factory.dart.direct.expect
index 8aeb2df1c6aed0af9b7ac3f920766db61b69f8a0..29aea3d2428d1908bfab25d8ec5242b4181a387c 100644
--- a/pkg/front_end/testcases/rasta/generic_factory.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/generic_factory.dart.direct.expect
@@ -3,17 +3,17 @@ import self as self;
 import "dart:core" as core;
 
 class C1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C3 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }