Implement type variables in mixin applications.

pkg/front_end/lib/src/fasta/kernel/kernel_target.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_target;
 
 import 'dart:async' show
     Future;
 
 import 'dart:io' show
@@ skipping 326 matching lines @@
     return null;
   }
 
   void installDefaultSupertypes(List<SourceClassBuilder> builders) {
     Class objectClass = this.objectClass;
     for (SourceClassBuilder builder in builders) {
       Class cls = builder.target;
       if (cls != objectClass) {
         cls.supertype ??= objectClass.asRawSupertype;
       }
+      if (builder.isMixinApplication) {
+        cls.mixedInType = builder.mixedInType.buildSupertype();
+      }
     }
     ticker.logMs("Installed Object as implicit superclass");
   }
 
   void installDefaultConstructors(List<SourceClassBuilder> builders) {
     Class objectClass = this.objectClass;
     for (SourceClassBuilder builder in builders) {
       if (builder.target != objectClass) {
         installDefaultConstructor(builder);
       }
     }
     ticker.logMs("Installed default constructors");
   }
 
   KernelClassBuilder get objectClassBuilder {
     return loader.coreLibrary.exports["Object"];
   }
 
   Class get objectClass => objectClassBuilder.cls;
 
   /// If [builder] doesn't have a constructors, install the defaults.
   void installDefaultConstructor(SourceClassBuilder builder) {
-    if (!builder.constructors.isEmpty) return;
+    if (builder.isMixinApplication || builder.constructors.isNotEmpty) return;
     /// Quotes below are from [Dart Programming Language Specification, 4th
     /// Edition](http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-408.pdf):
     if (builder is NamedMixinApplicationBuilder) {
       /// >A mixin application of the form S with M; defines a class C with
       /// >superclass S.
       /// >...
 
       /// >Let LM be the library in which M is declared. For each generative
       /// >constructor named qi(Ti1 ai1, . . . , Tiki aiki), i in 1..n of S
       /// >that is accessible to LM , C has an implicitly declared constructor
@@ skipping 196 matching lines @@
     superclass = superclass.superclass;
   }
   for (Constructor constructor in superclass.constructors) {
     if (constructor.name.name.isEmpty) {
       return constructor.function.requiredParameterCount == 0 ?
           constructor : null;
     }
   }
   return null;
 }