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

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

Index: pkg/front_end/lib/src/fasta/builder/builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/builder.dart b/pkg/front_end/lib/src/fasta/builder/builder.dart
index 01a120239cc90b3b13db272569221b77850eee36..5e5058775b76b93e8bf8ac95cc087a9c1c904c46 100644
--- a/pkg/front_end/lib/src/fasta/builder/builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/builder.dart
@@ -87,6 +87,15 @@ abstract class Builder {
   // this a const class.
   Builder next;
 
+  /// The values of [parent], [charOffset], and [fileUri] aren't stored. We
+  /// need to evaluate the memory impact of doing so, but want to ensure the
+  /// information is always provided.
+  Builder(Builder parent, int charOffset, Uri fileUri);
+
+  int get charOffset => -1;
+
+  Uri get fileUri => null;
+
   /// Resolve types (lookup names in scope) recorded in this builder and return
   /// the number of types resolved.
   int resolveTypes(Builder parent) => 0;
@@ -125,7 +134,7 @@ abstract class Builder {
     } else {
       print("${library.uri}: Note: '$name' is imported from both "
           "'${getUri(this)}' and '${getUri(other)}'.");
-      return library.buildAmbiguousBuilder(name, this, other);
+      return library.buildAmbiguousBuilder(name, this, other, charOffset);
     }
     if (isLocal) {
       print("${library.uri}: Note: local definition of '$name' hides imported "