Improve compile-time error handling.

pkg/front_end/lib/src/fasta/builder/library_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.library_builder;
 
 import '../combinator.dart' show
     Combinator;
 
 import '../errors.dart' show
@@ skipping 29 matching lines @@
   Map<String, Builder> get members;
 
   // TODO(ahe): Move this to SourceLibraryBuilder.
   Scope get scope;
 
   Map<String, Builder> get exports;
 
   LibraryBuilder(Uri fileUri)
       : super(null, -1, fileUri);
 
-  Builder addBuilder(String name, Builder builder);
+  Builder addBuilder(String name, Builder builder, int charOffset);
 
   void addExporter(LibraryBuilder exporter, List<Combinator> combinators,
       int charOffset) {
     exporters.add(new Export(exporter, this, combinators, charOffset));
   }
 
-  void addCompileTimeError(int charOffset, Object message) {
-    InputError error = new InputError(uri, charOffset, message);
+  void addCompileTimeError(int charOffset, Object message, [Uri fileUri]) {
+    fileUri ??= this.fileUri;
+    InputError error = new InputError(fileUri, charOffset, message);
     compileTimeErrors.add(error);
     print(error.format());
   }
 
   bool addToExportScope(String name, Builder member);
 
   void addToScope(String name, Builder member);
 
   InvalidTypeBuilder buildAmbiguousBuilder(
       String name, Builder builder, Builder other, int charOffset);
 
   int finishStaticInvocations() => 0;
 }