Fix crash in DillClassBuilder.isAbstract.

pkg/front_end/lib/src/fasta/kernel/body_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.body_builder;
 
 import 'package:front_end/src/fasta/parser/parser.dart' show
     FormalParameterType,
     optional;
 
@@ skipping 1628 matching lines @@
       arguments.types.addAll(typeArguments);
     }
 
     String errorName;
     if (type is ClassBuilder) {
       Builder b = type.findConstructorOrFactory(name);
       Member target;
       if (b == null) {
         // Not found. Reported below.
       } else if (b.isConstructor) {
-        if (type.cls.isAbstract) {
+        if (type.isAbstract) {
           // TODO(ahe): Generate abstract instantiation error.
         } else {
           target = b.target;
         }
       } else if (b.isFactory) {
         target = getRedirectionTarget(b.target);
         if (target == null) {
           push(buildCompileTimeError(
                   "Cyclic definition of factory '${name}'.",
                   token.charOffset));
           return;
         }
       }
       if (target is Constructor ||
           (target is Procedure && target.kind == ProcedureKind.Factory)) {
         push(buildStaticInvocation(
                 target, arguments, isConst: optional("const", token)));
         return;
       } else {
-        errorName = debugName(type.cls.name, name);
+        errorName = debugName(type.name, name);
       }
     } else {
       errorName = debugName(getNodeName(type), name);
     }
     errorName ??= name;
     push(throwNoSuchMethodError(errorName, arguments, token.charOffset));
   }
 
   @override
   void handleConstExpression(Token token) {
@@ skipping 1021 matching lines @@
   } else if (node is TypeDeclarationBuilder) {
     return node.name;
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisPropertyAccessor) {
     return node.name.name;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }