Throw ArgumentError if packageRoot is null.

dart/tests/compiler/dart2js/package_root_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that the compiler can handle imports when package root has not been set.
 
 library dart2js.test.package_root;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 12 matching lines @@
 
 import 'package:foo/foo.dart';
 
 main() {}
 ''',
 };
 
 void runCompiler(Uri main) {
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
+  Uri packageRoot = script.resolve('./packages/');
 
   var provider = new MemorySourceFileProvider(MEMORY_SOURCE_FILES);
   var handler = new FormattingDiagnosticHandler(provider);
   var errors = [];
 
   void diagnosticHandler(Uri uri, int begin, int end, String message,
                          Diagnostic kind) {
     if (kind == Diagnostic.ERROR) {
       errors.add(message);
     }
     handler(uri, begin, end, message, kind);
   }
 
 
   EventSink<String> outputProvider(String name, String extension) {
     if (name != '') throw 'Attempt to output file "$name.$extension"';
     return new NullSink('$name.$extension');
   }
 
   Compiler compiler = new Compiler(provider,
                                    outputProvider,
                                    diagnosticHandler,
                                    libraryRoot,
-                                   null,
+                                   packageRoot,
                                    [],
                                    {});
 
   asyncTest(() => compiler.run(main).then((_) {
     Expect.equals(1, errors.length);
-    Expect.equals("Cannot resolve 'package:foo/foo.dart'. "
-                  "Package root has not been set.",
-                  errors[0]);
+    Expect.isTrue(
+        errors[0].startsWith(
+            "Can't read 'package:foo/foo.dart' (Error reading "));
   }));
 }
 
 void main() {
   runCompiler(Uri.parse('memory:main.dart'));
   runCompiler(Uri.parse('package:foo/foo.dart'));
 }