Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference_new/infer_instance_accessor_ref.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field self::B b = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get c() → self::C
     return null;
   set c(self::C value) → void {}
 }
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     : super self::C::•()
     ;
 }
 static field self::A a = new self::A::•();
 static field self::C x = self::a.{self::A::b}.{self::B::c};
 static field self::C y = let final dynamic #t1 = self::a.{self::A::b} in let final dynamic #t2 = #t1.{self::B::c} in #t2.{core::Object::==}(null) ? #t1.{self::B::c} = new self::D::•() : #t2;
 static method main() → dynamic {}