Add native extensions to the Dart shell, on the linux platform.

runtime/bin/dartutils.h

Index: runtime/bin/dartutils.h
diff --git a/runtime/bin/dartutils.h b/runtime/bin/dartutils.h
index abb705a44df3a5b5c50197f17eac45bd0a0f595a..89a0bd8031ea135d9be3f60afc7a69361a9e1c60 100644
--- a/runtime/bin/dartutils.h
+++ b/runtime/bin/dartutils.h
@@ -68,6 +68,7 @@ class DartUtils {
                                      const char* name,
                                      const char* val);
   static bool IsDartSchemeURL(const char* url_name);
+  static bool IsDartExtensionSchemeURL(const char* url_name);
   static bool IsDartIOLibURL(const char* url_name);
   static bool IsDartJsonLibURL(const char* url_name);
   static bool IsDartUriLibURL(const char* url_name);
@@ -86,6 +87,7 @@ class DartUtils {
   static bool PostInt32(Dart_Port port_id, int32_t value);
 
   static const char* kDartScheme;
+  static const char* kDartExtensionScheme;
   static const char* kBuiltinLibURL;
   static const char* kCoreLibURL;
   static const char* kCoreImplLibURL;