Enables GN build on windows

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("..")
 
 template("invoke_dart") {
   assert(defined(invoker.outputs), "invoke_dart must specify outputs")
   extra_deps = []
   if (defined(invoker.deps)) {
@@ skipping 11 matching lines @@
   }
 
   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")
-    # 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,
     ] + extra_inputs
 
     outputs = invoker.outputs
 
     script = "$_dart_root/tools/dart_for_gn.py"
     args = [
       dart,
@@ skipping 35 matching lines @@
     abs_output = rebase_path(output)
     main_file = rebase_path(main_dart)
 
     args = [
       "--packages=$dot_packages",
       "--snapshot=$abs_output",
       main_file
     ]
   }
 }