Add member localHostname() to the Dart IO Platform class.

runtime/bin/platform.dart

Index: runtime/bin/platform.dart
diff --git a/runtime/bin/platform.dart b/runtime/bin/platform.dart
index 145f3011102892710cf18d009478dea187ac57f6..69b57853ffb85a5573927f02c636e3bc14c25fec 100644
--- a/runtime/bin/platform.dart
+++ b/runtime/bin/platform.dart
@@ -28,4 +28,9 @@ interface Platform default _Platform {
    * operating system.
    */
   String operatingSystem();
+
+  /**
+   * Get the local hostname for the system.
+   */
+  String localHostname();
 }