Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field = null;
   constructor •() → void
     : super core::Object::•()
     ;
   method foo() → dynamic
     return 87;
 }
+abstract class Object&Mixin extends core::Object implements self::Mixin {
+  field dynamic field = null;
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  method foo() → dynamic
+    return 87;
+}
 class Foo extends self::Object&Mixin {
   constructor •() → void
     : super self::Object&Mixin::•()
     ;
   method foo() → dynamic
     return this.{=self::Object&Mixin::foo}();
   method bar() → dynamic
     return this.{=self::Object&Mixin::field};
 }
-abstract class Object&Mixin extends core::Object implements self::Mixin {
-  field dynamic field = null;
-  constructor •() → void
-    : super core::Object::•()
-    ;
-  method foo() → dynamic
-    return 87;
-}
 static method main() → dynamic {
   dynamic f = new self::Foo::•();
   f.field = 42;
   core::print(f.bar());
 }