Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/type_variable_as_super.dart.outline.expect

Index: pkg/front_end/testcases/type_variable_as_super.dart.outline.expect
diff --git a/pkg/front_end/testcases/type_variable_as_super.dart.outline.expect b/pkg/front_end/testcases/type_variable_as_super.dart.outline.expect
index d8208123e000ad499f4f381b727c9b9db642228c..2d8fc872f444b431ab8f2aba837084ab5e92e9a2 100644
--- a/pkg/front_end/testcases/type_variable_as_super.dart.outline.expect
+++ b/pkg/front_end/testcases/type_variable_as_super.dart.outline.expect
@@ -3,7 +3,7 @@ import self as self;
 import "dart:core" as core;
 
 abstract class A<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 abstract class B<T extends core::Object> extends core::Object {
@@ -11,7 +11,7 @@ abstract class B<T extends core::Object> extends core::Object {
     ;
 }
 class C<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic