[dart:io] Adds Platform.ansiSupported

sdk/lib/io/platform_impl.dart

Index: sdk/lib/io/platform_impl.dart
diff --git a/sdk/lib/io/platform_impl.dart b/sdk/lib/io/platform_impl.dart
index 5743b9ef0684f6e9142b53246c587c76161b2888..f7637f17cacbbfe0145e60b060ca42d515066122 100644
--- a/sdk/lib/io/platform_impl.dart
+++ b/sdk/lib/io/platform_impl.dart
@@ -11,6 +11,7 @@ class _Platform {
   external static _localHostname();
   external static _executable();
   external static _resolvedExecutable();
+  external static bool _ansiSupported();
 
   /**
    * Retrieve the entries of the process environment.
@@ -45,6 +46,7 @@ class _Platform {
   static int get numberOfProcessors => _numberOfProcessors();
   static String get pathSeparator => _pathSeparator();
   static String get operatingSystem => _operatingSystem();
+  static bool get ansiSupported => _ansiSupported();
   static Uri script;
 
   static String get localHostname {