Handle local files better in server mode

lib/src/server/server.dart

 // Copyright (c) 2015, 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.
 
 /// Development server that compiles Dart to JS on the fly.
 library dev_compiler.src.server;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ skipping 236 matching lines @@
         // TODO(jmesserly): this logic is duplicated in a few places
         this._entryPath = compiler.options.sourceOptions.useImplicitHtml
             ? SourceResolverOptions.implicitHtmlFile
             : entryPath;
 
   Future start() async {
     // Create output directory if needed. shelf_static will fail otherwise.
     var out = new Directory(outDir);
     if (!await out.exists()) await out.create(recursive: true);
 
-    var mainHandler =
+    var generatedHandler =
         shelf_static.createStaticHandler(outDir, defaultDocument: _entryPath);
     var sourceHandler = shelf_static.createStaticHandler(compiler.inputBaseDir,
         serveFilesOutsidePath: true);
+    // TODO(vsm): Is there a better builtin way to compose these handlers?
     var topLevelHandler = (shelf.Request request) {
       var path = request.url.path;
-      if (path.endsWith('.dart')) {
-        return sourceHandler(request);
-      } else {
-        return mainHandler(request);
+      // Prefer generated code
+      var response = generatedHandler(request);
+      if (response.statusCode == 404) {
+        // Fall back on original sources
+        response = sourceHandler(request);
       }
+      return response;
     };
 
     var handler = const shelf.Pipeline()
         .addMiddleware(rebuildAndCache)
         .addHandler(topLevelHandler);
     await shelf.serve(handler, host, port);
     print('Serving $_entryPath at http://$host:$port/');
     // Give the compiler a head start. This is not needed for correctness,
     // but will likely speed up the first load. Regardless of whether compile
     // succeeds we should still start the server.
@@ skipping 42 matching lines @@
   Source resolveAbsolute(Uri uri, [Uri actualUri]) {
     var src = resolver.resolveAbsolute(uri, actualUri);
     return src.exists() ? src : null;
   }
 
   Uri restoreAbsolute(Source source) => resolver.restoreAbsolute(source);
 }
 
 final _log = new Logger('dev_compiler.src.server');
 final _earlyErrorResult = new CheckerResults(const [], true);