Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/builder/class_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/class_builder.dart b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
index a908ecbb03e4f7f286f762fe6a67b1d0ac3a5e02..33f0d176fc93ca2e4b4c220599303c288c9e3207 100644
--- a/pkg/front_end/lib/src/fasta/builder/class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
@@ -56,8 +56,14 @@ abstract class ClassBuilder<T extends TypeBuilder, R>
   /// superclass.
   bool get isMixinApplication => mixedInType != null;
 
+  bool get isNamedMixinApplication {
+    return isMixinApplication && super.isNamedMixinApplication;
+  }
+
   T get mixedInType;
 
+  void set mixedInType(T mixin);
+
   List<ConstructorReferenceBuilder> get constructorReferences => null;
 
   LibraryBuilder get library {
@@ -138,8 +144,9 @@ abstract class ClassBuilder<T extends TypeBuilder, R>
       builder = t.builder;
       arguments = t.arguments ?? const [];
       if (builder is ClassBuilder) {
-        variables = builder.typeVariables;
-        supertype = builder.supertype;
+        ClassBuilder cls = builder;
+        variables = cls.typeVariables;
+        supertype = cls.supertype;
       }
     }
 
@@ -155,7 +162,8 @@ abstract class ClassBuilder<T extends TypeBuilder, R>
         }
         supertype = t.supertype;
       } else {
-        internalError("Superclass not found.", fileUri, charOffset);
+        internalError("Superclass not found '${superclass.fullNameForErrors}'.",
+            fileUri, charOffset);
       }
       if (variables != null) {
         Map<TypeVariableBuilder, TypeBuilder> directSubstitutionMap =