Get binstubs working on Windows.

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

 import 'dart:io';
 import 'package:args/args.dart';
 import 'package:analyzer/src/services/formatter_impl.dart';
 import 'package:async_await/async_await.dart' as async_await;
 import 'package:path/path.dart' as p;
 final sourceDir = p.dirname(p.dirname(p.fromUri(Platform.script)));
 final sourceUrl = p.toUri(sourceDir).toString();
 final generatedDir = p.join(p.dirname(sourceDir), 'pub_generated');
 bool hadFailure = false;
 bool verbose = false;
@@ skipping 14 matching lines @@
           'Unexpected arguments: ${rest.skip(1).join(" ")}.');
     }
     buildDir = rest.first;
   } on FormatException catch (ex) {
     stderr.writeln(ex);
     stderr.writeln();
     stderr.writeln(
         "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();
   var match = _commitPattern.firstMatch(readme);
   if (match == null) {
     stderr.writeln("Could not find compiler commit hash in README.md.");
     exit(1);
   }
   lastCommit = match[0];
   var numFiles = 0;
@@ skipping 26 matching lines @@
   }
   if (currentCommit != lastCommit) {
     readme = readme.replaceAll(_commitPattern, currentCommit);
     _writeFile(readmePath, readme);
     if (verbose) print("Updated README.md");
   }
   if (numCompiled > 0) _generateSnapshot(buildDir);
   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);
   source = _translateAsyncAwait(sourcePath, source);
   if (source != null) source = _fixDart2jsImports(sourcePath, source, destPath);
   if (source == null) {
     _deleteFile(destPath);
   } else {
     _writeFile(destPath, source);
   }
@@ skipping 37 matching lines @@
 void _deleteFile(String path) {
   try {
     new File(path).deleteSync();
   } on IOException catch (ex) {}
 }
 void _writeFile(String path, String contents) {
   try {
     new File(path).writeAsStringSync(contents);
   } on IOException catch (ex) {}
 }