- Fix precompilation test.

tests/standalone/precompilation_dart2js_test.dart

 // Copyright (c) 2015, 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 generating and running a simple precompiled snapshot of this script.
 
 import 'dart:io';
 
 main(List args) {
   if (args.length > 0 && args[0] == "--hello") {
@@ skipping 25 matching lines @@
     cc_flags = "-m32";
   } else if (Platform.version.contains("arm")) {
     cc_flags = "";
   } else if (Platform.version.contains("mips")) {
     cc_flags = "-EL";
   } else {
     print("Architecture not supported: ${Platform.version}");
     return;
   }
 
-  var abs_package_root = new File(Platform.packageRoot).absolute.path;
+  var pkgRoot = Uri.parse(Platform.packageRoot);
+  var abs_package_root = new File.fromUri(pkgRoot).absolute.path;
   var dart_executable =
       Directory.current.path + Platform.pathSeparator + Platform.executable;
   Directory tmp;
   try {
     tmp = Directory.current.createTempSync("temp_precompilation_test");
     var exec = "${dart_executable}_no_snapshot";
     var args = ["--package-root=$abs_package_root",
                 "--gen-precompiled-snapshot",
                 "${abs_package_root}compiler/src/dart2js.dart"];
     print("$exec ${args.join(' ')}");
@@ skipping 42 matching lines @@
       throw "Precompiled binary failed.";
     }
     print(result.stdout);
     if (!result.stdout.contains("Dart-to-JavaScript compiler")) {
       throw "Precompiled binary output mismatch.";
     }
   } finally {
     tmp?.deleteSync(recursive: true);
   }
 }