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

tests/standalone/io/platform_test.dart

Index: tests/standalone/io/platform_test.dart
diff --git a/tests/standalone/io/platform_test.dart b/tests/standalone/io/platform_test.dart
index e3ee4d43780f0905b42637276ed4352876aaf596..bd154aead235a6401ab973bed612d6d250466e45 100644
--- a/tests/standalone/io/platform_test.dart
+++ b/tests/standalone/io/platform_test.dart
@@ -25,6 +25,11 @@ test() {
   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;