Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/builder/type_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/type_builder.dart b/pkg/front_end/lib/src/fasta/builder/type_builder.dart
index 8d212ef0bd7b50c7e972bf8aee77bc30d8a438df..b266e0fc373e17b0eed655554781e0d21322ecd8 100644
--- a/pkg/front_end/lib/src/fasta/builder/type_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/type_builder.dart
@@ -39,12 +39,4 @@ abstract class TypeBuilder extends Builder {
     printOn(sb);
     return "$sb";
   }
-
-  TypeBuilder computeMixin(
-      Uri fileUri, int charOffset, LibraryBuilder library) {
-    library.addCompileTimeError(
-        charOffset, "The type '$fullNameForErrors' can't be mixed in.",
-        fileUri: fileUri);
-    return this;
-  }
 }