Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/type_cast.dart.strong.expect

Index: pkg/front_end/testcases/inference/type_cast.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/type_cast.dart.strong.expect b/pkg/front_end/testcases/inference/type_cast.dart.strong.expect
index f48f34d9e73f8f982f18a64638f8cc68b5d9d3b5..6407324a47f14733fb968c6ab508a2228cc01808 100644
--- a/pkg/front_end/testcases/inference/type_cast.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/type_cast.dart.strong.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::•()
     ;
   method foo() → dynamic {}