Get binstubs working on Windows.

sdk/lib/_internal/pub_generated/bin/async_compile.dart

Index: sdk/lib/_internal/pub_generated/bin/async_compile.dart
diff --git a/sdk/lib/_internal/pub_generated/bin/async_compile.dart b/sdk/lib/_internal/pub_generated/bin/async_compile.dart
index 2f76c64baacf6945660702739d24059cf75ac1a8..15acba1ca63cf33ba2b775d4a33e75aa5bc150ba 100644
--- a/sdk/lib/_internal/pub_generated/bin/async_compile.dart
+++ b/sdk/lib/_internal/pub_generated/bin/async_compile.dart
@@ -32,15 +32,7 @@ void main(List<String> arguments) {
         "Usage: dart async_compile.dart [--verbose] [--force] <build dir>");
     exit(64);
   }
-  var result = Process.runSync(
-      "git",
-      ["rev-parse", "HEAD"],
-      workingDirectory: p.join(sourceDir, "../../../../third_party/pkg/async_await"));
-  if (result.exitCode != 0) {
-    stderr.writeln("Could not get Git revision of async_await compiler.");
-    exit(1);
-  }
-  var currentCommit = result.stdout.trim();
+  var currentCommit = _getCurrentCommit();
   var readmePath = p.join(generatedDir, "README.md");
   var lastCommit;
   var readme = new File(readmePath).readAsStringSync();
@@ -87,6 +79,23 @@ void main(List<String> arguments) {
   if (verbose) print("Compiled $numCompiled out of $numFiles files");
   if (hadFailure) exit(1);
 }
+String _getCurrentCommit() {
+  var command = "git";
+  var args = ["rev-parse", "HEAD"];
+  if (Platform.operatingSystem == "windows") {
+    command = "cmd";
+    args = ["/c", "git"]..addAll(args);
+  }
+  var result = Process.runSync(
+      command,
+      args,
+      workingDirectory: p.join(sourceDir, "../../../../third_party/pkg/async_await"));
+  if (result.exitCode != 0) {
+    stderr.writeln("Could not get Git revision of async_await compiler.");
+    exit(1);
+  }
+  return result.stdout.trim();
+}
 void _compile(String sourcePath, String source, String destPath) {
   var destDir = new Directory(p.dirname(destPath));
   destDir.createSync(recursive: true);