Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/inference/try_catch.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/try_catch.dart.strong.expect b/pkg/front_end/testcases/inference/try_catch.dart.strong.expect
index e84ac0cc855285249bf38d7af6cfb09459a41e9a..3dee6da5d12042fbb760d902df20dd9018ff4487 100644
--- a/pkg/front_end/testcases/inference/try_catch.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/try_catch.dart.strong.expect
@@ -3,17 +3,17 @@ import self as self;
 import "dart:core" as core;
 
 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::•()
     ;
 }
 class E extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }