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:kernel/ast.dart'
     show Class, Constructor, Supertype, TreeNode, setParents;
 
 import '../errors.dart' show internalError;
@@ skipping 26 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 84 matching lines @@
 
   void addSyntheticConstructor(Constructor constructor) {
     String name = constructor.name.name;
     cls.constructors.add(constructor);
     constructor.parent = cls;
     DillMemberBuilder memberBuilder = new DillMemberBuilder(constructor, this);
     memberBuilder.next = constructorScopeBuilder[name];
     constructorScopeBuilder.addMember(name, memberBuilder);
   }
 }