added package_root flag to standalone dart vm

runtime/bin/builtin.dart

 // Copyright (c) 2012, 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("builtin");
 #import("dart:uri");
 
 void print(arg) {
   _Logger._printString(arg.toString());
 }
@@ skipping 14 matching lines @@
 
 // Code to deal with URI resolution for the standalone binary.
 // For Windows we need to massage the paths a bit according to
 // http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx
 var _is_windows;
 
 // The URI that the entrypoint script was loaded from. Remembered so that
 // package imports can be resolved relative to it.
 Uri _entrypoint;
 
+// The directory to look in to resolve "package:" scheme URIs.
+String _packageRoot;
+
 void _logResolution(String msg) {
   final enabled = false;
   if (enabled) {
     _Logger._printString(msg);
   }
 }
 
+String _setPackageRoot(String packageRoot) {
+  // TODO(mattsh) - refactor windows drive and path handling code
+  // so it can be used here if needed.
+  _packageRoot = packageRoot;
+}
+
 String _resolveScriptUri(String cwd, String scriptName, bool windows) {
   _is_windows = windows;
   _logResolution("# Current working directory: $cwd");
   _logResolution("# ScriptName: $scriptName");
   if (windows) {
     // Convert
     // C:\one\two\three
     // to
     // /C:/one/two/three
     cwd = "/${cwd.replaceAll('\\', '/')}";
@@ skipping 82 matching lines @@
 String _filePathFromPackageUri(Uri uri) {
   if (uri.domain != '') {
     var path = (uri.path != '') ? '${uri.domain}${uri.path}' : uri.domain;
     var right = 'package:$path';
     var wrong = 'package://$path';
 
     throw "URIs using the 'package:' scheme should look like " +
           "'$right', not '$wrong'.";
   }
 
-  var path = _entrypoint.resolve('packages/${uri.path}').path;
+  var path;
+  if (_packageRoot !== null) {
+    path = "${_packageRoot}${uri.path}";
+  } else {
+    path = _entrypoint.resolve('packages/${uri.path}').path;
+  }
+
   _logResolution("# Package: $path");
   return path;
 }