Rewrite mixin application handling in Fasta.

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

Index: pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
index 48863d761d6c12747854c7cb77f0bceb9715091e..8c241096b98cc2e9f205a27cf8513ceb6c8f2591 100644
--- a/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
@@ -10,6 +10,14 @@ class Mixin extends 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::•()
@@ -19,14 +27,6 @@ class Foo extends self::Object&Mixin {
   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;