Rewrite mixin application handling in Fasta.

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

Index: pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
index dc735b036e9f60cad4589e0374ef1adbd3b705e7..8116f3fa5b4880dedf59cf82ef2c75c5cf15ba96 100644
--- a/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
@@ -14,11 +14,6 @@ class Mixin extends core::Object {
   method foo() → dynamic
     return core::print("foo");
 }
-class Sub extends self::Base&Mixin {
-  constructor •() → void
-    : super self::Base&Mixin::•()
-    ;
-}
 abstract class Base&Mixin extends self::Base implements self::Mixin {
   constructor •() → void
     : super self::Base::•()
@@ -26,6 +21,11 @@ abstract class Base&Mixin extends self::Base implements self::Mixin {
   method foo() → dynamic
     return core::print("foo");
 }
+class Sub extends self::Base&Mixin {
+  constructor •() → void
+    : super self::Base&Mixin::•()
+    ;
+}
 static method main() → dynamic {
   new self::Sub::•().foo();
 }