Add 'Server' and 'User-Agent' default header fields.

sdk/lib/io/options.dart

Index: sdk/lib/io/options.dart
diff --git a/sdk/lib/io/options.dart b/sdk/lib/io/options.dart
index 91a8e5841e6e46d1743d3608919b00a9e19f6e4f..ff409ffcd89de414470f7e3da635b70e6dd500d3 100644
--- a/sdk/lib/io/options.dart
+++ b/sdk/lib/io/options.dart
@@ -41,6 +41,12 @@ abstract class Options {
    * string is returned.
    */
   String get script;
+
+
+  /**
+   * Returns the version of the current dart runtime.
+   */
+  String get version;
 }
 
 class _OptionsImpl implements Options {
@@ -60,6 +66,8 @@ class _OptionsImpl implements Options {
     return _nativeScript;
   }
 
+  String get version;
+
   List<String> _arguments = null;
 
   // This arguments singleton is written to by the embedder if applicable.