Rewrite mixin application handling in Fasta.

pkg/front_end/lib/src/fasta/dill/dill_member_builder.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.dill_member_builder;
 
 import 'package:kernel/ast.dart'
     show Constructor, Field, Member, Procedure, ProcedureKind;
 
 import '../errors.dart' show internalError;
@@ skipping 35 matching lines @@
   bool get isSetter => identical(ProcedureKind.Setter, kind);
 
   bool get isOperator => identical(ProcedureKind.Operator, kind);
 
   bool get isFactory => identical(ProcedureKind.Factory, kind);
 
   bool get isRedirectingGenerativeConstructor {
     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) {
   int modifier = member.isAbstract ? abstractMask : 0;
   modifier |= member.isExternal ? externalMask : 0;
   if (member is Field) {
     modifier |= member.isConst ? constMask : 0;
     modifier |= member.isFinal ? finalMask : 0;
     modifier |= member.isStatic ? staticMask : 0;
   } else if (member is Procedure) {
     modifier |= member.isConst ? constMask : 0;
     modifier |= member.isStatic ? staticMask : 0;
   } else if (member is Constructor) {
     modifier |= member.isConst ? constMask : 0;
   } else {
     internalError("Unhandled: ${member.runtimeType}");
   }
   return modifier;
 }