GN: Fall back on dart_bootstrap where appropriate.

runtime/vm/BUILD.gn

Index: runtime/vm/BUILD.gn
diff --git a/runtime/vm/BUILD.gn b/runtime/vm/BUILD.gn
index fe20b99c3c4256c9bd3ffe4816187dd95f54a814..99c2bfb256a6d41f05123d0a60c868085aa40c3a 100644
--- a/runtime/vm/BUILD.gn
+++ b/runtime/vm/BUILD.gn
@@ -438,7 +438,8 @@ if (!defined(is_fuchsia) || !is_fuchsia) {
     # Build the patched sdk out of the concatenated patches and the special
     # libraries.
     action(target_name) {
-      deps = concatenation_target_names
+      deps = [ "../bin:dart_bootstrap($host_toolchain)" ] +
+             concatenation_target_names
 
       patches_dir = "$target_gen_dir/patches"
       patched_sdk_dir = "$root_out_dir/patched_sdk"
@@ -478,7 +479,17 @@ if (!defined(is_fuchsia) || !is_fuchsia) {
         "${patched_sdk_dir}/lib/core/core.dart",
       ]
 
+      dart_out_dir = get_label_info("../bin:dart_bootstrap($host_toolchain)",
+                                    "root_out_dir")
+      if (is_win) {
+        dart_bootstrap = rebase_path("$dart_out_dir/dart_bootstrap.exe")

[Cutch, 2016/11/01 19:33:52]

This `if (is_win) { } else { }` pattern is repeated, is there no
$platform_executable_suffix equivalent ? Maybe add one?

[zra, 2016/11/01 19:59:28]

Added to invoke_dart.gni

+      } else {
+        dart_bootstrap = rebase_path("$dart_out_dir/dart_bootstrap")
+      }
+
       args = [
+        "--dart-executable",
+        dart_bootstrap,
         "vm",
         rebase_path("../../sdk"),
         rebase_path(patches_dir, root_build_dir),