Fix a test crash with Dart 1.12.

test/io.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.
 
 library test.test.io;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
@@ skipping 34 matching lines @@
 }
 
 /// Runs Dart.
 ProcessResult runDart(List<String> args, {String workingDirectory,
     Map<String, String> environment}) {
   var allArgs = Platform.executableArguments.map((arg) {
     // The package root might be relative, so we need to make it absolute if
     // we're going to run in a different working directory.
     if (!arg.startsWith("--package-root=")) return arg;
     return "--package-root=" +
-        p.absolute(arg.substring("--package-root=".length));
+        p.absolute(p.fromUri(arg.substring("--package-root=".length)));
   }).toList()..addAll(args);
 
   // TODO(nweiz): Use ScheduledProcess once it's compatible.
   return new _NormalizedProcessResult(Process.runSync(
       p.absolute(Platform.executable), allArgs,
       workingDirectory: workingDirectory, environment: environment));
 }
 
 /// Runs Pub.
 ProcessResult runPub(List<String> args, {String workingDirectory,
@@ skipping 67 matching lines @@
 
   _NormalizedProcessResult(ProcessResult inner)
       : _inner = inner,
         stdout = Platform.isWindows
             ? inner.stdout.replaceAll("\r\n", "\n")
             : inner.stdout,
         stderr = Platform.isWindows
             ? inner.stderr.replaceAll("\r\n", "\n")
             : inner.stderr;
 }