Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect

Index: pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect
index bd4dca530b03e68790d0eb700eb4592a6b074a8c..33c7e1d9debbab1e1597d1ab165975a5e94e3994 100644
--- a/pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect
@@ -15,14 +15,9 @@ class M1 extends core::Object {
     : super core::Object::•()
     ;
 }
-class C extends self::A&M1 {
-  constructor •(core::bool v1, core::num v2) → void
-    : super self::A&M1::•(v1, v2)
-    ;
-}
-abstract class A&M1 extends self::A implements self::M1 {
+class C extends self::A implements self::M1 {
   field core::num v2 = 0;
-  constructor •(final core::bool v1, final core::num v2) → void
+  constructor •(core::bool v1, core::num v2) → void
     : super self::A::•(v1, v2)
     ;
 }