Set fileUri when creating library builders.

pkg/front_end/lib/src/fasta/source/source_library_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.source_library_builder;
 
 import 'package:kernel/ast.dart' show
     AsyncMarker,
     ProcedureKind;
 
@@ skipping 44 matching lines @@
       <ConstructorReferenceBuilder>[];
 
   final List<LibraryBuilder> parts = <LibraryBuilder>[];
 
   final List<Import> imports = <Import>[];
 
   final Map<String, Builder> exports = <String, Builder>{};
 
   final Scope scope = new Scope(<String, Builder>{}, null, isModifiable: false);
 
+  final Uri fileUri;
+
   String name;
 
   String partOf;
 
-  Uri fileUri;
-
   List<MetadataBuilder> metadata;
 
   Map<String, MemberBuilder> classMembers;
 
   // TODO(ahe): Rename this. It's not just for classes.
   List<T> classTypes;
 
-  SourceLibraryBuilder(this.loader);
+  SourceLibraryBuilder(this.loader, this.fileUri);
 
   Uri get uri;
 
   bool get isPart => partOf != null;
 
   T addInterfaceType(String name, List<T> arguments);
 
   T addMixinApplication(T supertype, List<T> mixins);
 
   T addType(T type) {
@@ skipping 37 matching lines @@
 
   void addPart(List<MetadataBuilder> metadata, String path) {
     Uri resolvedUri;
     Uri newFileUri;
     if (uri.scheme == "dart") {
       resolvedUri = new Uri(scheme: "dart", path: "${uri.path}/$path");
       newFileUri = fileUri.resolve(path);
     } else {
       newFileUri = resolvedUri = resolve(path);
     }
-    LibraryBuilder part = loader.read(resolvedUri);
-    if (part is SourceLibraryBuilder) {
-      part.fileUri ??= newFileUri;
-    }
-    parts.add(part);
+    parts.add(loader.read(resolvedUri, newFileUri));
   }
 
   void addPartOf(List<MetadataBuilder> metadata, String name) {
     partOf = name;
   }
 
   ClassBuilder addClass(List<MetadataBuilder> metadata,
       int modifiers, String name,
       List<TypeVariableBuilder> typeVariables, T supertype,
       List<T> interfaces);
@@ skipping 186 matching lines @@
   }
 
   int resolveConstructors(_) {
     int count = 0;
     members.forEach((String name, Builder member) {
       count += member.resolveConstructors(this);
     });
     return count;
   }
 }