Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/store_load.dart.strong.expect

Index: pkg/front_end/testcases/store_load.dart.strong.expect
diff --git a/pkg/front_end/testcases/store_load.dart.strong.expect b/pkg/front_end/testcases/store_load.dart.strong.expect
index c06ea707f23b6c0bd443e0a14ca67a7f1c7e7797..1c51fda63b0bd0a46ddc0d48450694c7630a91ee 100644
--- a/pkg/front_end/testcases/store_load.dart.strong.expect
+++ b/pkg/front_end/testcases/store_load.dart.strong.expect
@@ -4,23 +4,23 @@ import "dart:core" as core;
 
 class Foo extends core::Object {
   field dynamic _field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class FooValue extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Bar extends core::Object {
   field dynamic _field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class BarValue extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }