Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method test() → void {
     dynamic v1 = super.foo(self::f<dynamic>());
     dynamic v2 = super.bar;
     dynamic v3 = super.[](0);
     dynamic v4 = super.bar = self::f<dynamic>();
     dynamic v5 = let final dynamic #t1 = 0 in let final dynamic #t2 = self::f<dynamic>() in let final dynamic #t3 = super.[]=(#t1, #t2) in #t2;
   }
 }
 static method f<T extends core::Object>() → self::f::T
   return null;
 static method main() → dynamic {}