Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method f<T extends core::Object>() → self::D<self::C::f::T>
     return null;
 }
 class D<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static field self::D<core::int> f = new self::C::•().{self::C::f}<core::int>();
 static method main() → dynamic {}