Port parser and scanner fixes from rasta branch.

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
@@ skipping 38 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";
     } else {
       return message;
     }
   }
 }
 
 class Crash {
   final Uri uri;
@@ skipping 75 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.";
   }
 }