Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000068.dart.direct.expect

Index: pkg/front_end/testcases/rasta/issue_000068.dart.direct.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000068.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000068.dart.direct.expect
index b66e51d771d4ba7923058aa423e6bc01601921c8..cfe6ec7217d12e92255b322d80e3c09ae3031c8a 100644
--- a/pkg/front_end/testcases/rasta/issue_000068.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000068.dart.direct.expect
@@ -4,22 +4,22 @@ import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 
 class G<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }