I have no idea if this is actually what we want to do, but we are hitting the long path issue on wi…

pkg/polymer/lib/src/build/runner.dart

 // Copyright (c) 2013, 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.
 
 /// Definitions used to run the polymer linter and deploy tools without using
 /// pub serve or pub deploy.
 library polymer.src.build.runner;
 
 import 'dart:async';
 import 'dart:convert';
@@ skipping 198 matching lines @@
     }
   });
 }
 
 /// Emits all outputs of [barback] and copies files that we didn't process (like
 /// dependent package's libraries).
 Future _emitAllFiles(Barback barback, BarbackOptions options) {
   return barback.getAllAssets().then((assets) {
     // Delete existing output folder before we generate anything
     var dir = new Directory(options.outDir);
-    if (dir.existsSync()) dir.deleteSync(recursive: true);
+    // We get very long filenames which can cause us to not be able to delete
+    // using the normal dart:io deleteSync(recursive: true) on windows
+    if (dir.existsSync()) {
+      if (Platform.operatingSystem == 'windows' ) {
+        var result = Process.runSync('rmdir', ['/q', '/s', options.outDir]);
+        if (result.exitCode != 0) {
+          throw "Could not delete $dir, output was: \n"
+                 "stdout: ${result.stdout} \n"
+                 "stderr: ${result.stderr}";
+        }
+      } else {
+        dir.deleteSync(recursive: true);
+      }
+    }
     return _emitPackagesDir(options)
       .then((_) => _emitTransformedFiles(assets, options))
       .then((_) => _addPackagesSymlinks(assets, options))
       .then((_) => assets);
   });
 }
 
 Future _emitTransformedFiles(AssetSet assets, BarbackOptions options) {
   // Copy all the assets we transformed
   var futures = [];
@@ skipping 131 matching lines @@
     output.write(entry.span.getLocationMessage(entry.message,
           useColors: useColors,
           color: levelColor));
   }
   return output.toString();
 }
 
 const String _RED_COLOR = '\u001b[31m';
 const String _MAGENTA_COLOR = '\u001b[35m';
 const String _NO_COLOR = '\u001b[0m';