Rewrite mixin application handling in Fasta.

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

Index: pkg/front_end/testcases/rasta/issue_000048.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000048.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000048.dart.outline.expect
index 219634af5066342fd60db32d7fe314ad24d3fb35..a7bda3a80dd3e1c3fed964cb7f449cdf17d7b030 100644
--- a/pkg/front_end/testcases/rasta/issue_000048.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000048.dart.outline.expect
@@ -13,12 +13,10 @@ class M1 extends core::Object {
   constructor •() → void
     ;
 }
-class C extends self::A&M1 {
+class C = self::A with self::M1 {
   constructor •(core::bool v1, core::num v2) → void
     : super self::A::•(v1, v2)
     ;
 }
-abstract class A&M1 = self::A with self::M1 {
-}
 static method main() → dynamic
   ;