Rewrite mixin application handling in Fasta.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart
index 5967433f9bf25ebb868d3d80c826782f629f9307..7b2f8cb53a2ddf59a04c6215ecf1254626bc8f33 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_class_builder.dart
@@ -281,4 +281,10 @@ abstract class KernelClassBuilder
       }
     }
   }
+
+  String get fullNameForErrors {
+    return isMixinApplication
+        ? "${supertype.fullNameForErrors} with ${mixedInType.fullNameForErrors}"
+        : name;
+  }
 }