Canonicalize mixin applications by their name

pkg/front_end/lib/src/fasta/kernel/kernel_library_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.kernel_library_builder;
 
 import 'package:kernel/ast.dart';
 
 import 'package:kernel/clone.dart' show
     CloneVisitor;
@@ skipping 43 matching lines @@
     NamedMixinApplicationBuilder,
     PrefixBuilder,
     ProcedureBuilder,
     TypeBuilder,
     TypeVariableBuilder;
 
 class KernelLibraryBuilder
     extends SourceLibraryBuilder<KernelTypeBuilder, Library> {
   final Library library;
 
-  final List<Class> mixinApplicationClasses = <Class>[];
+  final Map<String, SourceClassBuilder> mixinApplicationClasses =
+      <String, SourceClassBuilder>{};
 
   final List<List> argumentsWithMissingDefaultValues = <List>[];
 
   final List<KernelProcedureBuilder> nativeMethods = <KernelProcedureBuilder>[];
 
   final List<KernelTypeVariableBuilder> boundlessTypeVariables =
       <KernelTypeVariableBuilder>[];
 
   KernelLibraryBuilder(Uri uri, Uri fileUri, Loader loader)
       : library = new Library(uri, fileUri: relativizeUri(fileUri)),
@@ skipping 269 matching lines @@
     }
     boundlessTypeVariables.clear();
     return count;
   }
 
   @override
   void includePart(covariant KernelLibraryBuilder part) {
     super.includePart(part);
     nativeMethods.addAll(part.nativeMethods);
     boundlessTypeVariables.addAll(part.boundlessTypeVariables);
-    mixinApplicationClasses.addAll(part.mixinApplicationClasses);
+    assert(mixinApplicationClasses.isEmpty);
   }
 }
 
 bool isConstructorName(String name, String className) {
   if (name.startsWith(className)) {
     if (name.length == className.length) return true;
     if (name.startsWith(".", className.length)) return true;
   }
   return false;
 }