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

pkg/front_end/lib/src/fasta/builder/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.builder;
 
 import '../errors.dart' show
     internalError;
 
 export 'class_builder.dart' show
@@ skipping 69 matching lines @@
     LibraryBuilder;
 
 abstract class Builder {
   /// Used when multiple things with the same name are declared within the same
   /// parent. Only used for declarations, not for scopes.
   ///
   // TODO(ahe): Move to member builder or something. Then we can make
   // 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;
 
   /// Resolve constructors (lookup names in scope) recorded in this builder and
   /// return the number of constructors resolved.
   int resolveConstructors(Builder parent) => 0;
 
   /// Look for methods with the same name as their enclosing class and convert
   /// them to constructors. Return the number of methods converted to
@@ skipping 18 matching lines @@
         getUri(this)?.scheme != "dart") {
       preferred = this;
       hidden = other;
     } else if (getUri(this)?.scheme == "dart" &&
         getUri(other)?.scheme != "dart") {
       preferred = other;
       hidden = this;
     } 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 "
           "version from '${getUri(other)}'.");
     } else {
       print("${library.uri}: import of '$name' (from '${getUri(preferred)}') "
           "hides imported version from '${getUri(hidden)}'.");
     }
     return preferred;
   }
@@ skipping 30 matching lines @@
 
   static Uri getUri(Builder builder) {
     if (builder == null) return internalError("Builder is null.");
     while (builder != null) {
       if (builder is LibraryBuilder) return builder.uri;
       builder = builder.parent;
     }
     return internalError("No library parent.");
   }
 }