Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field dynamic a;
   field dynamic b;
   field dynamic c;
   field dynamic d;
   final field dynamic f;
-  constructor •() → self::A
+  default constructor •() → self::A
     : super core::Object::•();
   get e() → dynamic
     return null;
   set g(dynamic _) → dynamic {}
   get h() → dynamic
     return null;
   set h(dynamic _) → dynamic {}
   get i() → dynamic
     return null;
   operator [](dynamic _) → dynamic
     return null;
   operator []=(dynamic a, dynamic b) → dynamic {}
   operator ~() → dynamic
     return 117;
   operator unary-() → dynamic
     return 117;
   operator ==(dynamic other) → dynamic
     return true;
   method m() → void {}
   method n() → void {}
   set n(dynamic _) → dynamic {}
 }
 class B extends self::A {
   final field dynamic d;
-  constructor •() → self::B
+  default constructor •() → self::B
     : super self::A::•();
   get b() → dynamic
     return null;
   set c(dynamic x) → dynamic {}
   set i(dynamic x) → dynamic {}
 }
 class C extends self::B {
-  constructor •() → self::C
+  default constructor •() → self::C
     : super self::B::•();
   method test() → dynamic {
     super.{self::A::~}();
     self::use(super.{self::A::~}());
     super.{self::A::unary-}();
     self::use(super.{self::A::unary-}());
     invalid-expression;
     self::use(invalid-expression);
     super.{self::A::==}(87);
     self::use(super.{self::A::==}(87));
@@ skipping 195 matching lines @@
     self::use(throw core::_unresolvedSuperGetterError(this, #n, <dynamic>[], <dynamic, dynamic>{}, null));
   }
 }
 static method use(dynamic x) → dynamic {
   if(x.==(new core::DateTime::now().millisecondsSinceEpoch))
     throw "Shouldn't happen";
 }
 static method main() → dynamic {
   new self::C::•().test();
 }