Add FileSystemEntity.absolutePath and .isAbsolute properties.

tests/standalone/io/file_absolute_path_test.dart

+// Copyright (c) 2013, 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.
+//
+// Dart test program for testing FileSystemEntity.absolute
+
+import "package:expect/expect.dart";
+import 'dart:io';
+
+main() {
+  if (Platform.isWindows) {
+    testWindows();
+    try {
+      Directory.current = 'C:\\';
+    } catch (e) {
+      return;
+    }
+    testWindows();
+  } else {
+    testPosix();
+    Directory.current = '.';
+    testPosix();
+    Directory.current = '/';
+    testPosix();
+
+  }
+}
+
+testWindows() {
+  String current = Directory.current.path;
+  for (String relative in ['abd', '..', '.', 'efg/hij', 'abc/']) {
+    if (current.endsWith('\\')) {
+      Expect.equals(new File(relative).absolute.path, '$current$relative');
+    } else {
+      Expect.equals(new File(relative).absolute.path, '$current\\$relative');
+    }
+    Expect.isTrue(new File(relative).absolute.isAbsolute);
+  }
+  for (String absolute in ['c:/abd', 'D:\\rf', '\\\\a_share\\folder',
+                           '\\\\?\\c:\\prefixed\path\\']) {
+    Expect.isTrue(new File(absolute).absolute.path == absolute);
+    Expect.isTrue(new File(absolute).absolute.isAbsolute);
+  }
+}
+
+testPosix() {
+  String current = Directory.current.path;
+  print(Directory.current.path);
+  for (String relative in ['abd', '..', '.', 'efg/hij', 'abc/']) {
+    if (current.endsWith('/')) {
+      Expect.equals(new File(relative).absolute.path, '$current$relative');
+    } else {
+      Expect.equals(new File(relative).absolute.path, '$current/$relative');
+    }
+    Expect.isTrue(new File(relative).absolute.isAbsolute);
+    Expect.equals(new Directory(relative).absolute.path,
+                  new Link(relative).absolute.path);
+    Expect.isTrue(new File(relative).absolute is File);
+    Expect.isTrue(new Directory(relative).absolute is Directory);
+    Expect.isTrue(new Link(relative).absolute is Link);
+  }
+  for (String absolute in ['/abd', '/', '/./..\\', '/efg/hij', '/abc/']) {
+    Expect.equals(new File(absolute).absolute.path, absolute);
+    Expect.isTrue(new File(absolute).absolute.isAbsolute);
+  }
+}