Ensure locations are always provided, but don't store them yet.

pkg/front_end/lib/src/fasta/kernel/kernel_mixin_application_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.kernel_mixin_application_builder;
 
 import 'package:kernel/ast.dart' show
     Class,
     InterfaceType,
     Supertype;
 
 import 'kernel_builder.dart' show
     KernelTypeBuilder,
     MixinApplicationBuilder;
 
+import '../util/relativize.dart' show
+    relativizeUri;
+
 class KernelMixinApplicationBuilder
     extends MixinApplicationBuilder<KernelTypeBuilder>
     implements KernelTypeBuilder {
+  final int charOffset;
+
+  final String relativeFileUri;
+
   Supertype builtType;
 
   KernelMixinApplicationBuilder(KernelTypeBuilder supertype,
-      List<KernelTypeBuilder> mixins)
-      : super(supertype, mixins);
+      List<KernelTypeBuilder> mixins, int charOffset, Uri fileUri)
+      : charOffset = charOffset,
+        relativeFileUri = relativizeUri(fileUri),
+        super(supertype, mixins, charOffset, fileUri);
 
   InterfaceType build() => buildSupertype().asInterfaceType;
 
   Supertype buildSupertype() {
     if (builtType != null) return builtType;
     Supertype supertype =
         this.supertype.buildSupertype()?.classNode?.asRawSupertype;
     if (supertype == null) {
       return null;
     }
     for (KernelTypeBuilder builder in mixins) {
       Supertype mixin = builder.buildSupertype()?.classNode?.asRawSupertype;
       if (mixin == null) {
         return null;
       }
       Class application = new Class(
           name: "${supertype.classNode.name}&${mixin.classNode.name}",
           isAbstract: true,
           supertype: supertype,
           mixedInType: mixin,
-          typeParameters: null); // TODO(ahe): Compute these.
+          typeParameters: null, // TODO(ahe): Compute these.
+          fileUri: relativeFileUri);
+      application.fileOffset = charOffset;
       // TODO(ahe): Use asThisSupertype instead and translate type variables.
       supertype = application.asRawSupertype;
     }
     builtType = supertype;
     return builtType;
   }
 }