Implement type variables in mixin applications.

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index 8704e05e34da78ca963c71eaf574f9380abb4cb5..f32be7b4a4868cdb62e075da1179569d222e7f61 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -344,6 +344,9 @@ class KernelTarget extends TargetImplementation {
       if (cls != objectClass) {
         cls.supertype ??= objectClass.asRawSupertype;
       }
+      if (builder.isMixinApplication) {
+        cls.mixedInType = builder.mixedInType.buildSupertype();
+      }
     }
     ticker.logMs("Installed Object as implicit superclass");
   }
@@ -366,7 +369,7 @@ class KernelTarget extends TargetImplementation {
 
   /// 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) {