Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 import "dart:async" as asy;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }
 class C extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }
 static method main() → dynamic async {
   asy::Future<self::B> b = asy::Future::value<self::B>(new self::B::•());
   asy::Future<self::C> c = asy::Future::value<self::C>(new self::C::•());
   core::List<asy::Future<self::A>> lll = <asy::Future<self::A>>[b, c];
   core::List<self::A> result = await asy::Future::wait<self::A>(lll);
   core::List<self::A> result2 = await asy::Future::wait<self::A>(<asy::Future<self::A>>[b, c]);
   core::List<self::A> list = result;
   list = result2;
 }