Make Platform.executable return a fully qualified path the the executable

tests/standalone/io/platform_test.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.
 
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 test() {
   Expect.isTrue(Platform.numberOfProcessors > 0);
   var os = Platform.operatingSystem;
   Expect.isTrue(os == "android" || os == "linux" || os == "macos" ||
       os == "windows");
   Expect.equals(Platform.isLinux, Platform.operatingSystem == "linux");
   Expect.equals(Platform.isMacOS, Platform.operatingSystem == "macos");
   Expect.equals(Platform.isWindows, Platform.operatingSystem == "windows");
   Expect.equals(Platform.isAndroid, Platform.operatingSystem == "android");
   var sep = Platform.pathSeparator;
   Expect.isTrue(sep == '/' || (os == 'windows' && sep == '\\'));
   var hostname = Platform.localHostname;
   Expect.isTrue(hostname is String && hostname != "");
   var environment = Platform.environment;
   Expect.isTrue(environment is Map<String, String>);
   Expect.isTrue(Platform.executable.contains('dart'));
+  if (!Platform.isWindows) {
+    Expect.isTrue(Platform.executable.startsWith('/'));
+  } else {
+    Expect.equals(Platform.executable.substring(1, 3), ':\\');

[Lasse Reichstein Nielsen, 2015/05/20 10:48:43]

That's slightly optimistic in general, but probably fine for this test.
Windows can use UNC names "\\foo\bar\baz" and long names "\\?\.." and even
combinations.

Can we make a test where the executable is at a UNC-named location?

[Søren Gjesse, 2015/05/20 11:14:24]

True I am assuming that tests (both locally and on the bots) are running off a
location referred to by a drive letter. I don't think we have a share we can use
for testing.

Added a comment. If it gives problems I can work more on it.

+  }
   // Move directory to be sure script is correct.
   var oldDir = Directory.current;
   Directory.current = Directory.current.parent;
   Expect.isTrue(Platform.script.path.
                 endsWith('tests/standalone/io/platform_test.dart'));
   Expect.isTrue(Platform.script.toFilePath().startsWith(oldDir.path));
   // Restore dir.
   Directory.current = oldDir;
   Directory packageRoot = new Directory(Platform.packageRoot);
   Expect.isTrue(packageRoot.existsSync());
@@ skipping 79 matching lines @@
   Expect.throws(() => checkValidVersion('x.y.z'));
 }
 
 main() {
   // This tests assumes paths relative to dart main directory
   Directory.current = Platform.script.resolve('../../..').toFilePath();
   test();
   testIsolate();
   testVersion();
 }