Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 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 Foo<T extends self::A> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → void {
   self::Foo<self::B> foo = new self::Foo::•<self::B>();
 }