GN: Don't depend on dart_bootstrap if there is a usable prebuilt sdk

build/prebuilt_dart_sdk.gni

+# Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+import("executable_suffix.gni")
+
+_dart_root = rebase_path("..")
+
+_prebuilt_dart_exe = "$_dart_root/tools/sdks/$host_os/dart-sdk/bin/dart"
+
+# Our gn_run_binary swallows output unless there is an error.
+_prebuilt_dart_exe_trial = exec_script("gn_run_binary.py",
+    [_prebuilt_dart_exe, "--version"], "string")
+if (_prebuilt_dart_exe_trial == "") {
+  prebuilt_dart_exe_works = true
+} else {
+  prebuilt_dart_exe_works = false
+}