Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/cycles.dart.direct.expect

Index: pkg/front_end/testcases/cycles.dart.direct.expect
diff --git a/pkg/front_end/testcases/cycles.dart.direct.expect b/pkg/front_end/testcases/cycles.dart.direct.expect
index 950f38228bdf4bef611d67d37b07fc03a7544bc8..fdeae6e588076d26c59d14258772df1691f6b408 100644
--- a/pkg/front_end/testcases/cycles.dart.direct.expect
+++ b/pkg/front_end/testcases/cycles.dart.direct.expect
@@ -3,22 +3,22 @@ import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }