Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference_new/infer_assign_to_property_custom.dart.outline.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   operator +(dynamic other) → core::int
     ;
   operator -(dynamic other) → core::double
     ;
 }
 class B extends core::Object {
   field self::A a;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static field dynamic v_prefix_pp;
 static field dynamic v_prefix_mm;
 static field dynamic v_postfix_pp;
 static field dynamic v_postfix_mm;
 static method main() → dynamic
   ;