added package_root flag to standalone dart vm

runtime/bin/builtin.dart

Index: runtime/bin/builtin.dart
diff --git a/runtime/bin/builtin.dart b/runtime/bin/builtin.dart
index e0108096f6c8934346edbf6b103b0ea3dfe26d75..c2b98e272eb3341b6a4c999e988f3a44f83cf25d 100644
--- a/runtime/bin/builtin.dart
+++ b/runtime/bin/builtin.dart
@@ -32,6 +32,9 @@ var _is_windows;
 // 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) {
@@ -39,6 +42,12 @@ void _logResolution(String 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: $cwd");
@@ -141,7 +150,13 @@ String _filePathFromPackageUri(Uri uri) {
           "'$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;
 }