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

pkg/front_end/lib/src/fasta/builder/mixed_accessor.dart

Index: pkg/front_end/lib/src/fasta/builder/mixed_accessor.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/mixed_accessor.dart b/pkg/front_end/lib/src/fasta/builder/mixed_accessor.dart
index dafe137e1d9d9498e7811c97c66ee40641a21dc1..42297a6dd3d16cd35918a40e5d8bd86e460564b2 100644
--- a/pkg/front_end/lib/src/fasta/builder/mixed_accessor.dart
+++ b/pkg/front_end/lib/src/fasta/builder/mixed_accessor.dart
@@ -5,14 +5,17 @@
 library fasta.mixed_accessor;
 
 import 'builder.dart' show
-    Builder;
+    Builder,
+    LibraryBuilder;
 
 /// Represents the import of a getter and setter from two different libraries.
 class MixedAccessor extends Builder {
   final Builder getter;
   final Builder setter;
 
-  MixedAccessor(this.getter, this.setter) {
+  MixedAccessor(this.getter, this.setter, LibraryBuilder parent)
+      : super(parent, -1, // Synthetic element has no charOffset.
+          parent.fileUri) {
     next = getter;
   }
 }