Rewrite mixin application handling in Fasta.

pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect

Index: pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
index b279d88d5afd10cd57aecd086241e70bd62d7149..ed5fd1f40f045c525820711ee5ddd50c3c8e94d4 100644
--- a/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
@@ -12,11 +12,11 @@ class Mixin extends core::Object {
   method foo() → dynamic
     ;
 }
+abstract class Base&Mixin = self::Base with self::Mixin {
+}
 class Sub extends self::Base&Mixin {
   constructor •() → void
     ;
 }
-abstract class Base&Mixin = self::Base with self::Mixin {
-}
 static method main() → dynamic
   ;