Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/generic_factory.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class C1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C3 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class A<T extends core::Object> extends core::Object {
   static field dynamic _redirecting# = <dynamic>[self::A::a, self::A::b, self::A::c];
   constructor internal() → void
     : super core::Object::•()
     ;
   static factory a<T extends core::Object>() → self::A<self::A::a::T>
     let dynamic #redirecting_factory = self::B::a in invalid-expression;
@@ skipping 17 matching lines @@
     : super self::B::internal()
     ;
 }
 static method main() → dynamic {
   new self::C::•<self::C3>();
   new self::C::•<self::C3>();
   new self::C::•<self::C3>();
   new self::C::•<self::C3>();
   throw new core::NoSuchMethodError::_withType(null, #Missing, 32, <dynamic>[].toList(growable: false), <dynamic, dynamic>{}, null);
 }