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

pkg/front_end/lib/src/fasta/import.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.import;
 
 import 'builder/builder.dart' show
     Builder,
     LibraryBuilder,
     PrefixBuilder;
 
 import 'combinator.dart' show
     Combinator;
 
 typedef void AddToScope(String name, Builder member);
 
 class Import {
+  /// The library that is importing [imported];
+  final LibraryBuilder importer;
+
   /// The library being imported.
   final LibraryBuilder imported;
 
   final String prefix;
 
   final List<Combinator> combinators;
 
-  Import(this.imported, this.prefix, this.combinators);
+  final int charOffset;
+
+  final int prefixCharOffset;
+
+  Import(this.importer, this.imported, this.prefix, this.combinators,
+      this.charOffset, this.prefixCharOffset);
+
+  Uri get fileUri => importer.fileUri;
 
   void finalizeImports(LibraryBuilder importer) {
     AddToScope add;
     PrefixBuilder prefix;
     if (this.prefix == null) {
       add = importer.addToScope;
     } else {
-      prefix = new PrefixBuilder(this.prefix, <String, Builder>{}, importer);
+      prefix = new PrefixBuilder(
+          this.prefix, <String, Builder>{}, importer, prefixCharOffset);
       add = (String name, Builder member) {
         prefix.exports[name] = member;
       };
     }
     imported.exports.forEach((String name, Builder member) {
       if (combinators != null) {
         for (Combinator combinator in combinators) {
           if (combinator.isShow && !combinator.names.contains(name)) return;
           if (combinator.isHide && combinator.names.contains(name)) return;
         }
       }
       add(name, member);
     });
     if (prefix != null) {
       Builder existing = importer.addBuilder(prefix.name, prefix);
       if (existing == prefix) {
         importer.addToScope(prefix.name, prefix);
       }
     }
   }
 }