Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/void-methods.dart.strong.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   field core::List<dynamic> list = <dynamic>[1, 2, 3];
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
-  set first(dynamic x) → dynamic
+  set first(dynamic x) → void
     return let final dynamic #t1 = this.{self::Foo::list} in let final dynamic #t2 = 0 in let final dynamic #t3 = x in let final dynamic #t4 = #t1.{core::List::[]=}(#t2, #t3) in #t3;
   operator []=(dynamic x, dynamic y) → dynamic
     return let final dynamic #t5 = this.{self::Foo::list} in let final dynamic #t6 = x in let final dynamic #t7 = y in let final dynamic #t8 = #t5.{core::List::[]=}(#t6, #t7) in #t7;
   method clear() → void
     return this.{self::Foo::list}.{core::List::clear}();
 }
 static method main() → dynamic {
   new self::Foo::•().{self::Foo::first} = 4;
   new self::Foo::•().{self::Foo::[]=}(3, 4);
   new self::Foo::•().{self::Foo::clear}();
 }