Implement setPosition on File to seek to a byte position.

runtime/bin/file.cc

Index: runtime/bin/file.cc
diff --git a/runtime/bin/file.cc b/runtime/bin/file.cc
index 58817ea629538ad9744e439ed1897d01e9dfdac5..d1d61082d83389640866804cf0c6ecc237fc1c5a 100644
--- a/runtime/bin/file.cc
+++ b/runtime/bin/file.cc
@@ -215,6 +215,22 @@ void FUNCTION_NAME(File_Position)(Dart_NativeArguments args) {
 }
 
 
+void FUNCTION_NAME(File_SetPosition)(Dart_NativeArguments args) {
+  Dart_EnterScope();
+  intptr_t return_value = -1;
+  intptr_t value =
+      DartUtils::GetIntegerValue(Dart_GetNativeArgument(args, 0));
+  File* file = reinterpret_cast<File*>(value);
+  if (file != NULL) {
+    int64_t position =
+        DartUtils::GetIntegerValue(Dart_GetNativeArgument(args, 1));
+    return_value = file->SetPosition(position);
+  }
+  Dart_SetReturnValue(args, Dart_NewInteger(return_value));
+  Dart_ExitScope();
+}
+
+
 void FUNCTION_NAME(File_Length)(Dart_NativeArguments args) {
   Dart_EnterScope();
   intptr_t return_value = -1;