Throw ArgumentError if packageRoot is null.

dart/pkg/compiler/lib/src/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 1333 matching lines @@
   static const MessageKind AMBIGUOUS_LOCATION = const MessageKind(
       "'#{name}' is defined here.");
 
   static const MessageKind IMPORTED_HERE = const MessageKind(
       "'#{name}' is imported here.");
 
   static const MessageKind OVERRIDE_EQUALS_NOT_HASH_CODE = const MessageKind(
       "The class '#{class}' overrides 'operator==', "
       "but not 'get hashCode'.");
 
-  static const MessageKind PACKAGE_ROOT_NOT_SET = const MessageKind(
-      "Cannot resolve '#{uri}'. Package root has not been set.");
-
   static const MessageKind INTERNAL_LIBRARY_FROM = const MessageKind(
       "Internal library '#{resolvedUri}' is not accessible from "
       "'#{importingUri}'.");
 
   static const MessageKind INTERNAL_LIBRARY = const MessageKind(
       "Internal library '#{resolvedUri}' is not accessible.");
 
   static const MessageKind LIBRARY_NOT_FOUND = const MessageKind(
       "Library not found '#{resolvedUri}'.");
 
@@ skipping 1074 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }