Enables GN build on windows

utils/invoke_dart.gni

Index: utils/invoke_dart.gni
diff --git a/utils/invoke_dart.gni b/utils/invoke_dart.gni
index 287d94c633f31d1cf871aa3850cb8f520ab058eb..66705f712b2e50a6f55a8f0a435560dc8126713a 100644
--- a/utils/invoke_dart.gni
+++ b/utils/invoke_dart.gni
@@ -29,8 +29,11 @@ template("invoke_dart") {
 
     dart_out_dir = get_label_info(
         "$relative_dart_root/runtime/bin:dart($host_toolchain)", "root_out_dir")
-    # TODO(zra): Add platform specific executable suffix (e.g. .exe on win)
-    dart = rebase_path("$dart_out_dir/dart")
+    if (is_win) {
+      dart = rebase_path("$dart_out_dir/dart.exe")

[Cutch, 2016/10/04 15:58:48]

is there some kind ${executable_suffix} variable that could be used
unconditionally?

[zra, 2016/10/04 16:08:35]

Toolchain definitions know what the extension will be, but there's no way to
inspect the current toolchain other than asking its name. We could manually set
something up in BUILDCONFIG.gn, but then other GN builds (e.g. Flutter, Fuchsia)
would get confused by the symbol. This seems like the cleanest way,
unfortunately.

+    } else {
+      dart = rebase_path("$dart_out_dir/dart")
+    }
 
     inputs = [
       dart,