Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/mixin.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
+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");
+}
 class B extends self::Object&M1&M2 {
   constructor •(dynamic value) → void
     : super self::Object&M1&M2::•()
     ;
 }
 abstract class M1 extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic
@@ skipping 11 matching lines @@
     : super self::Object&M1&M2::•()
     ;
 }
 abstract class G1<T extends core::Object> extends core::Object {
   constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic
     return core::print(self::G1::T);
 }
-class D<S extends core::Object> extends self::D^G1<self::D::S> {
-  constructor •() → void
-    : super self::D^G1::•()
-    ;
-}
-abstract class Object&M1 extends core::Object implements self::M1 {
+abstract class Object&G1^^#T0<#T0 extends core::Object> extends core::Object implements self::G1<self::Object&G1^^#T0::#T0> {
   constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic
-    return core::print("M1");
+    return core::print(self::Object&G1^^#T0::#T0);
 }
-abstract class Object&M1&M2 extends self::Object&M1 implements self::M2 {
+class D<S extends core::Object> extends self::Object&G1^^#T0<self::D::S> {
   constructor •() → void
-    : super self::Object&M1::•()
+    : super self::Object&G1^^#T0::•()
     ;
-  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();
 }