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);
     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) {

[Jennifer Messerly, 2016/01/27 20:50:02]

this feels a little bit off wrong in the architecture. Shouldn't we be able to
chain request handlers without manually checking for 404?

My understanding of Shelf is that it should support multiple kinds of handlers:
https://pub.dartlang.org/packages/shelf

Anyway, not a big deal...

+        // 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);