Improve position information.

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:front_end/src/scanner/token.dart' show Token;
 
 import 'package:kernel/ast.dart' show ProcedureKind;
 
@@ skipping 131 matching lines @@
     imports.add(new Import(
         this,
         loader.read(resolve(uri), charOffset, accessor: this),
         deferred,
         prefix,
         combinators,
         charOffset,
         prefixCharOffset));
   }
 
-  void addPart(List<MetadataBuilder> metadata, String path) {
+  void addPart(List<MetadataBuilder> metadata, String path, int charOffset) {
     Uri resolvedUri;
     Uri newFileUri;
     if (uri.scheme == "dart") {
       resolvedUri = resolveRelativeUri(uri, Uri.parse(path));
       newFileUri = fileUri.resolve(path);
     } else {
       resolvedUri = uri.resolve(path);
       if (uri.scheme != "package") {
         newFileUri = fileUri.resolve(path);
       }
@@ skipping 415 matching lines @@
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 
   Scope toScope(Scope parent) {
     return new Scope(members, setters, parent, isModifiable: false);
   }
 }