[fasta] Add and correct even more offsets

pkg/front_end/lib/src/fasta/source/source_class_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_class_builder;
 
 import 'package:kernel/ast.dart'
     show Class, Constructor, Supertype, TreeNode, setParents;
 
 import '../errors.dart' show internalError;
@@ skipping 14 matching lines @@
         TypeVariableBuilder;
 
 import '../dill/dill_member_builder.dart' show DillMemberBuilder;
 
 import '../util/relativize.dart' show relativizeUri;
 
 Class initializeClass(
     Class cls, String name, LibraryBuilder parent, int charOffset) {
   cls ??= new Class(name: name);
   cls.fileUri ??= relativizeUri(parent.fileUri);
-  if (cls.fileOffset != TreeNode.noOffset) {
+  if (cls.fileOffset == TreeNode.noOffset) {
     cls.fileOffset = charOffset;
   }
   return cls;
 }
 
 class SourceClassBuilder extends KernelClassBuilder {
   final Class cls;
 
   final Map<String, Builder> constructors;
 
@@ skipping 96 matching lines @@
 Map<String, Builder> computeConstructors(Map<String, Builder> members) {
   Map<String, Builder> constructors = <String, Builder>{};
   members.forEach((String name, Builder builder) {
     if (builder is ProcedureBuilder &&
         (builder.isConstructor || builder.isFactory)) {
       constructors[name] = builder;
     }
   });
   return constructors;
 }