Canonicalize mixin applications by their name

pkg/front_end/test/fasta/mixin.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class B extends self::Object&M1&M2 {
   constructor •(dynamic value) → void
     : super self::Object&M1&M2::•()
     ;
 }
 abstract class M1 extends core::Object {
@@ skipping 34 matching lines @@
   method m() → dynamic
     return core::print("M1");
 }
 abstract class Object&M1&M2 extends self::Object&M1 implements self::M2 {
   constructor •() → void
     : super self::Object&M1::•()
     ;
   method m() → dynamic
     return core::print("M2");
 }
-abstract class Object&M1 extends core::Object implements self::M1 {
-  constructor •() → void
-    : super core::Object::•()
-    ;
-  method m() → dynamic
-    return core::print("M1");
-}
-abstract class Object&M1&M2 extends self::Object&M1 implements self::M2 {
-  constructor •() → void
-    : super self::Object&M1::•()
-    ;
-  method m() → dynamic
-    return core::print("M2");
-}
 abstract class D^G1<S extends core::Object> extends core::Object implements self::G1<self::D^G1::S> {
   constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic
     return core::print(self::D^G1::S);
 }
 static method main() → dynamic {
   new self::B::•(null).m();
   new self::C::•(null).m();
   new self::D::•<dynamic>().m();
   new self::D::•<core::int>().m();
   new self::D::•<core::List<core::int>>().m();
 }