Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/inference/type_cast.dart.outline.expect
diff --git a/pkg/front_end/testcases/inference/type_cast.dart.outline.expect b/pkg/front_end/testcases/inference/type_cast.dart.outline.expect
index c6ac78218b3a3acada6dcf15c060ca284f5acc73..aee9eb42b86795d0c5fd0df5f0d9de4f7d15aaa5 100644
--- a/pkg/front_end/testcases/inference/type_cast.dart.outline.expect
+++ b/pkg/front_end/testcases/inference/type_cast.dart.outline.expect
@@ -3,11 +3,11 @@ import self as self;
 import "dart:core" as core;
 
 class A<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class B<T extends core::Object> extends self::A<self::B::T> {
-  constructor •() → void
+  default constructor •() → void
     ;
   method foo() → dynamic
     ;