Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/assign_local.dart.direct.expect

Index: pkg/front_end/testcases/inference/assign_local.dart.direct.expect
diff --git a/pkg/front_end/testcases/inference/assign_local.dart.direct.expect b/pkg/front_end/testcases/inference/assign_local.dart.direct.expect
index e51ac1c3e210eb5750895cb72823655103b7b2e1..1b9f37c27d5e0af6052486f0bad2ca3628907964 100644
--- a/pkg/front_end/testcases/inference/assign_local.dart.direct.expect
+++ b/pkg/front_end/testcases/inference/assign_local.dart.direct.expect
@@ -3,12 +3,12 @@ import self as self;
 import "dart:core" as core;
 
 class A<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B<T extends core::Object> extends self::A<self::B::T> {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }