Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "./infer_type_regardless_of_declaration_order_or_cycles_b.dart" as inf;
 import "dart:core" as core;
 
 class C extends inf::B {
-  constructor •() → void
+  default constructor •() → void
     : super inf::B::•()
     ;
   get x() → core::int
     return null;
 }
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get x() → core::int
     return 0;
 }
 static method foo() → dynamic {
   core::int y = new self::C::•().{self::C::x};
   core::String z = new self::C::•().{self::C::x};
 }
 static method main() → dynamic {
   self::foo();
 }