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

runtime/vm/BUILD.gn

Index: runtime/vm/BUILD.gn
diff --git a/runtime/vm/BUILD.gn b/runtime/vm/BUILD.gn
index 7922cf0226f67b4d73c0424f53328a6999a84eee..285b0d52334eb964eeaa4b0836fdc82ec2c7efea 100644
--- a/runtime/vm/BUILD.gn
+++ b/runtime/vm/BUILD.gn
@@ -2,7 +2,8 @@
 # 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("../../utils/invoke_dart.gni")
+import("../../build/executable_suffix.gni")
+import("../../build/prebuilt_dart_sdk.gni")
 
 config("libdart_vm_config") {
   if (defined(is_fuchsia) && is_fuchsia) {
@@ -440,8 +441,10 @@ if (!defined(is_fuchsia) || !is_fuchsia) {
     # Build the patched sdk out of the concatenated patches and the special
     # libraries.
     action(target_name) {
-      deps = [ "../bin:dart_bootstrap($host_toolchain)" ] +
-             concatenation_target_names
+      deps = concatenation_target_names
+      if (!prebuilt_dart_exe_works) {
+        deps += [ "../bin:dart_bootstrap($host_toolchain)" ]
+      }
 
       patches_dir = "$target_gen_dir/patches"
       patched_sdk_dir = "$root_out_dir/patched_sdk"
@@ -481,14 +484,18 @@ if (!defined(is_fuchsia) || !is_fuchsia) {
         "${patched_sdk_dir}/lib/core/core.dart",
       ]
 
-      dart_out_dir = get_label_info("../bin:dart_bootstrap($host_toolchain)",
-                                    "root_out_dir")
-      dart_bootstrap =
-          rebase_path("$dart_out_dir/dart_bootstrap$dart_executable_suffix")
-
-      args = [
-        "--dart-executable",
-        dart_bootstrap,
+      args = []
+      if (!prebuilt_dart_exe_works) {
+        dart_out_dir = get_label_info("../bin:dart_bootstrap($host_toolchain)",
+                                      "root_out_dir")
+        dart_bootstrap =
+            rebase_path("$dart_out_dir/dart_bootstrap$executable_suffix")
+        args += [
+          "--dart-executable",
+          dart_bootstrap,
+        ]
+      }
+      args += [
         "vm",
         rebase_path("../../sdk"),
         rebase_path(patches_dir, root_build_dir),