Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object implements self::B {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get x() → core::int
     return self::f();
   set x(core::int value) → void {}
 }
 class B extends core::Object {
   field core::int x = 0;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method f() → dynamic
   return null;
 static method main() → dynamic {}