Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
index 1c666068d4390c8518a6d03acf7f27af99a819a3..688738807d9d9de6a8897c1c4a59a182bdc8756d 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
@@ -53,6 +53,13 @@ class DillMemberBuilder extends MemberBuilder {
     return isConstructor &&
         isRedirectingGenerativeConstructorImplementation(member);
   }
+
+  bool get isSynthetic {
+    // TODO(ahe): Kernel should eventually support a synthetic bit.
+    return isConstructor &&
+        name == "" &&
+        (charOffset == parent.charOffset || charOffset == -1);
+  }
 }
 
 int computeModifiers(Member member) {