[fasta] Add and correct even more offsets

pkg/front_end/lib/src/fasta/source/source_class_builder.dart

Index: pkg/front_end/lib/src/fasta/source/source_class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
index 017534bf16aaaadf96d3167b351b6abf180b6006..8990bb28868022aed31568c432513e14ca144dae 100644
--- a/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_class_builder.dart
@@ -32,7 +32,7 @@ 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;