Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/propagate_inference_transitively2.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field core::int x = 42;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
   field self::A a = new self::A::•();
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C extends core::Object {
   field self::B b = new self::B::•();
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends core::Object {
   field self::C c = new self::C::•();
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → void {
   self::D d1 = new self::D::•();
   core::print(d1.{self::D::c}.{self::C::b}.{self::B::a}.{self::A::x});
   self::D d2 = new self::D::•();
   core::print(d2.{self::D::c}.{self::C::b}.{self::B::a}.{self::A::x});
 }