Do not duplicate constructors for named mixin applications.

testcases/spec-mode/redirecting_factory.baseline.txt

 library;
 import self as self;
 import "dart:core" as core;
 
 abstract class FooBase<Tf extends core::Object> extends core::Object {
   abstract get x() → core::int;
 }
 abstract class Foo<T extends core::Object> extends core::Object implements self::FooBase<dynamic> {
 }
 class Bar<Sb extends core::Object, Tb extends core::Object> extends core::Object implements self::Foo<self::Bar::Tb> {
@@ skipping 27 matching lines @@
 }
 class Mixin<M extends core::Object> extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
 }
 class Mix<M extends core::Object> extends self::Base<self::Mix::M> implements self::Mixin<self::Mix::M> {
   constructor •() → void
     : super self::Base::•()
     ;
-  constructor •() → void
-    : super self::Base::•()
-    ;
 }
 static method main() → dynamic {
   core::print(new self::Bar::•<core::String, core::double>(4).x);
   new self::SimpleCaseImpl2::•<core::int, core::double>();
   new self::Mix::•<core::double>();
 }