Update SourceMapViewer to work on Windows.

tools/dart2js/sourceMapViewer/bin/source_map_viewer.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:io';
 import 'package:http_server/http_server.dart' as http_server;
 import 'package:route/server.dart';
 import 'package:path/path.dart';
 import 'package:http_server/http_server.dart';
 
 /*
  * This program serves a visualization of a JavaScript source map file generated
  * by dart2js or pub.
  *
  * Usage: dart source_map_viewer.dart <path to map file>.
  *
  * The default system browser is started and pointed to the viewer if
  * available.
  */
 
-Directory rootDir = null;
-String sourceMapFile;
+Uri sourceMapFile;
 
 void main(List<String> args) {
   if (args.length != 1) {
     print('One argument expected; the source map file.');
     exit(-1);
     return;
   }
 
   File mapFile = new File(args[0]);
   if (!mapFile.existsSync()) {
     print('Map file not found at ${args[0]}');
     exit(-2);
     return;
   }
 
-  sourceMapFile = basename(mapFile.path);
-  rootDir = mapFile.parent;
-  startServer(rootDir);
+  sourceMapFile = toUri(mapFile.path);
+  startServer();
 }
 
 // Sends the content of the file requested in the path parameter.
 void handleFile(HttpRequest request) {
   String path = request.uri.queryParameters["path"];
   if (path == null) {
     request.response.close();
     return;
   }
 
-  path = rootDir.path + '/' + path;
-  new File(Uri.parse(path).toFilePath()).openRead()
-    .pipe(request.response).catchError((e) {
-  print("Error: $e");
+  Uri uri = sourceMapFile.resolve(path);
+  new File.fromUri(uri).openRead().pipe(request.response).catchError((e) {
+    print("Error: $e");
     request.response.close();
   });
 }
 
 // Sends back the name of the source map file.
 void handleSourceMapFile(HttpRequest request) {
-  request.response.write(sourceMapFile);
+  request.response.write(basename(sourceMapFile.path));
   request.response.close();
 }
 
 // Starts an HttpServer rooted in [dir] with two special routes, /file and /map.
 //
 // /file takes a parameter [path] and serves the content of the specified file
 //  in path relative to [dir]
 //
 // /map serves the name of the map file such that its content can be requested
 // with a /file request as above.
-void startServer(Directory dir) {
-  rootDir = dir;
+void startServer() {
+  String root = fromUri(Platform.script.resolve('../build/web/'));
+  Directory directory = new Directory(root);
+  if (!directory.existsSync()) {
+    print("Directory '$root' does not exist. "
+          "Run 'pub build' to generate the output.");
+    exit(-1);
+  }
+
   // Use port 0 to get an ephemeral port.
   int port = 0;
   HttpServer.bind(InternetAddress.LOOPBACK_IP_V4, port).then((server) {
     port = server.port;
     print("Source mapping server is running on "
            "'http://${server.address.address}:$port/'");
     Router router = new Router(server)
                 ..serve('/file').listen(handleFile)
                 ..serve('/map').listen(handleSourceMapFile);
 
     // Set up default handler. This will serve files from our 'build'
     // directory. Disable jail root, as packages are local symlinks.
-    VirtualDirectory virDir = new http_server.VirtualDirectory(dir.path)
+    VirtualDirectory virDir = new http_server.VirtualDirectory(root)
       ..jailRoot = false
       ..allowDirectoryListing = true;
 
     virDir.directoryHandler = (dir, request) {
       // Redirect directory requests to index.html files.
       Uri indexUri = new Uri.file(dir.path).resolve('display.html');
       virDir.serveFile(new File(indexUri.toFilePath()), request);
     };
 
     // Add an error page handler.
@@ skipping 26 matching lines @@
     }
   }
 
   print('Starting browser: ${command} ${url}');
   Process.run(command, ['$url']).then((ProcessResult result) {
     if (result.exitCode != 0) {
       print(result.stderr);
     }
   });
 }