Add support for passing in precompiled JS.

lib/src/runner/browser/platform.dart

 // Copyright (c) 2016, 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:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:async/async.dart';
 import 'package:http_multi_server/http_multi_server.dart';
@@ skipping 106 matching lines @@
         _compilers = new CompilerPool(config) {
     var cascade = new shelf.Cascade()
         .add(_webSocketHandler.handler);
 
     if (_config.pubServeUrl == null) {
       cascade = cascade
           .add(_createPackagesHandler())
           .add(_jsHandler.handler)
           .add(createStaticHandler(_root))
           .add(_wrapperHandler);
+
+      if (config.precompiledPath != null) {
+        cascade = cascade.add(createStaticHandler(config.precompiledPath));
+      }
     }
 
     var pipeline = new shelf.Pipeline()
       .addMiddleware(nestingMiddleware(_secret))
       .addHandler(cascade.handler);
 
     _server.mount(pipeline);
   }
 
   /// Returns a handler that serves the contents of the "packages/" directory
@@ skipping 98 matching lines @@
         dartUrl = _config.pubServeUrl.resolve(
             "$suitePrefix.html.polymer.bootstrap.dart.browser_test.dart");
       } else {
         dartUrl = _config.pubServeUrl.resolve(
           '$suitePrefix.dart.browser_test.dart');
       }
 
       await _pubServeSuite(path, dartUrl, browser);
       suiteUrl = _config.pubServeUrl.resolveUri(p.toUri('$suitePrefix.html'));
     } else {
-      if (browser.isJS) await _compileSuite(path);
+      if (browser.isJS && !_precompiled(path)) await _compileSuite(path);
       if (_closed) return null;
       suiteUrl = url.resolveUri(p.toUri(
           p.withoutExtension(p.relative(path, from: _root)) + ".html"));
     }
 
     if (_closed) return null;
 
     // TODO(nweiz): Don't start the browser until all the suites are compiled.
     var browserManager = await _browserManagerFor(browser);
     if (_closed) return null;
 
     var suite = await browserManager.load(path, suiteUrl, metadata,
         mapper: browser.isJS ? _mappers[path] : null);
     if (_closed) return null;
     return suite;
   }
 
+  /// Returns whether the test at [path] has precompiled JS available underneath
+  /// `_config.precompiledPath`.
+  bool _precompiled(String path) {
+    if (_config.precompiledPath == null) return false;
+    var jsPath =
+        p.join(_config.precompiledPath, p.relative(path, from: _root)) +
+            ".browser_test.dart.js";
+    print("does $jsPath exist? ${new File(jsPath).existsSync()}");
+    return new File(jsPath).existsSync();
+  }
+
   StreamChannel loadChannel(String path, TestPlatform platform) =>
       throw new UnimplementedError();
 
   /// Loads a test suite at [path] from the `pub serve` URL [dartUrl].
   ///
   /// This ensures that only one suite is loaded at a time, and that any errors
   /// are exposed as [LoadException]s.
   Future _pubServeSuite(String path, Uri dartUrl, TestPlatform browser) {
     return _pubServePool.withResource(() async {
       var timer = new Timer(new Duration(seconds: 1), () {
@@ skipping 150 matching lines @@
     await Future.wait(futures);
 
     if (_config.pubServeUrl == null) {
       new Directory(_compiledDir).deleteSync(recursive: true);
     } else {
       _http.close();
     }
   });
   final _closeMemo = new AsyncMemoizer();
 }