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

utils/invoke_dart.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.
-
-_dart_root = rebase_path("..")
-
-if (defined(is_win) && is_win) {
-  dart_executable_suffix = ".exe"
-} else {
-  dart_executable_suffix = ""
-}
-
-template("invoke_dart") {
-  assert(defined(invoker.outputs), "invoke_dart must specify outputs")
-  extra_deps = []
-  if (defined(invoker.deps)) {
-    extra_deps += invoker.deps
-  }
-
-  extra_inputs = []
-  if (defined(invoker.inputs)) {
-    extra_inputs += invoker.inputs
-  }
-
-  extra_args = []
-  if (defined(invoker.args)) {
-    extra_args += invoker.args
-  }
-
-  action(target_name) {
-    relative_dart_root = rebase_path(_dart_root, ".")
-    deps =
-        extra_deps + [ "$relative_dart_root/runtime/bin:dart($host_toolchain)" ]
-
-    dart_out_dir =
-        get_label_info("$relative_dart_root/runtime/bin:dart($host_toolchain)",
-                       "root_out_dir")
-    dart = rebase_path("$dart_out_dir/dart$dart_executable_suffix")
-
-    inputs = [ dart ] + extra_inputs
-
-    outputs = invoker.outputs
-
-    script = "$_dart_root/tools/dart_for_gn.py"
-    args = [ dart ] + extra_args
-  }
-}
-
-template("application_snapshot") {
-  assert(defined(invoker.main_dart), "Must specify 'main_dart'")
-  assert(defined(invoker.training_args), "Must specify 'training_args'")
-  main_dart = invoker.main_dart
-  training_args = invoker.training_args
-  name = target_name
-  if (defined(invoker.name)) {
-    name = invoker.name
-  }
-  extra_deps = []
-  if (defined(invoker.deps)) {
-    extra_deps += invoker.deps
-  }
-  extra_inputs = []
-  if (defined(invoker.inputs)) {
-    extra_inputs += invoker.inputs
-  }
-  invoke_dart(target_name) {
-    deps = extra_deps + [ "$_dart_root/pkg:pkg_files_stamp" ]
-
-    inputs = extra_inputs + [
-               "$_dart_root/sdk/lib/_internal/sdk_library_metadata/lib/libraries.dart",
-               "$root_gen_dir/pkg_files.stamp",
-             ]
-
-    output = "$root_gen_dir/$name.dart.snapshot"
-    outputs = [
-      output,
-    ]
-
-    dot_packages = rebase_path("$_dart_root/.packages")
-    abs_output = rebase_path(output)
-    main_file = rebase_path(main_dart)
-
-    args = [
-             "--packages=$dot_packages",
-             "--snapshot=$abs_output",
-             "--snapshot-kind=app-jit",
-             main_file,
-           ] + training_args
-  }
-}