Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/source/source_class_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.source_class_builder;
 
 import 'package:front_end/src/fasta/builder/class_builder.dart'
     show ClassBuilder;
 
 import 'package:front_end/src/fasta/source/source_library_builder.dart'
@@ skipping 32 matching lines @@
     cls.fileOffset = charOffset;
   }
   return cls;
 }
 
 class SourceClassBuilder extends KernelClassBuilder {
   final Class cls;
 
   final List<ConstructorReferenceBuilder> constructorReferences;
 
-  final KernelTypeBuilder mixedInType;
+  KernelTypeBuilder mixedInType;
 
   SourceClassBuilder(
       List<MetadataBuilder> metadata,
       int modifiers,
       String name,
       List<TypeVariableBuilder> typeVariables,
       KernelTypeBuilder supertype,
       List<KernelTypeBuilder> interfaces,
       Scope scope,
       Scope constructors,
@@ skipping 91 matching lines @@
   }
 
   @override
   void prepareInitializerInference(
       SourceLibraryBuilder library, ClassBuilder currentClass) {
     scope.forEach((name, builder) {
       builder.prepareInitializerInference(library, this);
     });
   }
 }