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

pkg/front_end/lib/src/fasta/errors.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.errors;
 
 import 'dart:async' show
     Future;
 
 import 'dart:convert' show
     JSON;
 
 import 'dart:io' show
     ContentType,
     HttpClient,
     HttpClientRequest,
     SocketException,
     stderr;
 
 import 'colors.dart' show
     red;
 
+import 'util/relativize.dart' show
+    relativizeUri;
+
 const String defaultServerAddress = "http://127.0.0.1:59410/";
 
 /// Tracks if there has been a crash reported through [reportCrash]. Should be
 /// reset between each compilation by calling [resetCrashReporting].
 bool hasCrashed = false;
 
 /// Tracks the first source URI that has been read and is used as a fall-back
 /// for [reportCrash]. Should be reset between each compilation by calling
 /// [resetCrashReporting].
 Uri firstSourceUri;
@@ skipping 16 matching lines @@
   InputError(this.uri, int charOffset, this.error)
       : this.charOffset = charOffset ?? -1;
 
   toString() => "InputError: $error";
 
   String format() {
     // TODO(ahe): Colors need to be optional. Doesn't work well in Emacs or on
     // Windows.
     String message = red("Error: ${safeToString(error)}");
     if (uri != null) {
-      String uri = "${this.uri}";
-      String base = "${Uri.base}";
-      if (uri.startsWith(base)) {
-        uri = uri.substring(base.length);
-      }
       String position = charOffset == -1 ? "" : "$charOffset:";
-      return "${uri}:$position $message";
+      return "${relativizeUri(uri)}:$position $message";
     } else {
       return message;
     }
   }
 }
 
 class Crash {
   final Uri uri;
 
   final int charOffset;
@@ skipping 73 matching lines @@
   return new Future.error(error, trace);
 }
 
 String safeToString(Object object) {
   try {
     return "$object";
   } catch (e) {
     return "Error when converting ${object.runtimeType} to string.";
   }
 }