Add native extensions for the Dart shell to mac and windows

runtime/bin/extensions.h

Index: runtime/bin/extensions.h
diff --git a/runtime/bin/extensions.h b/runtime/bin/extensions.h
index fda8f1e4d41754e1f9591496fdd7d55f262b33ac..b62c7637afc1e18b8809311142cd527b99cee0e5 100644
--- a/runtime/bin/extensions.h
+++ b/runtime/bin/extensions.h
@@ -10,13 +10,19 @@
 
 class Extensions {
  public:
-  // TODO(whesse): Make loading extensions lazy, so dynamic library is loaded
-  // only when first native function is called.
   // TODO(whesse): Make extension load from a relative path relative to
   // the library it is in.  Currently loads from current working directory.
   static Dart_Handle LoadExtension(const char* extension_url,
-                                   Dart_Handle library);
+                                   Dart_Handle parent_library);
+
  private:
+  // The returned string must be freed.
+  static char* Concatenate(const char** strings);
+
+  // Platform-specific implementations.
+  static void* LoadExtensionLibrary(const char* library_name);
+  static void* ResolveSymbol(void* lib_handle, const char* symbol);
+
   DISALLOW_ALLOCATION();
   DISALLOW_IMPLICIT_CONSTRUCTORS(Extensions);
 };