Add package-config tests where test is running in Isolate.spawn-isolates.

tests/standalone/packages_file_test.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:io";
 import "dart:convert" show JSON;
 import "package:path/path.dart" as p;
 import "package:async_helper/async_helper.dart";
 
@@ skipping 227 matching lines @@
     String httpUri =
         "http://${httpServer.address.address}:${httpServer.port}/$id/";
 
     String fixPath(String path) {
       return path?.replaceAllMapped(fileHttpRegexp, (match) {
         if (path.startsWith("%file/", match.start)) return fileUri;
         return httpUri;
       });
     }
 
-    String fixPaths(Map dirs) {
+    void fixPaths(Map dirs) {
       for (var name in dirs.keys) {
         var value = dirs[name];
         if (value is Map) {
           Map subDir = value;
           fixPaths(subDir);
         } else {
           var newValue = fixPath(value);
           if (newValue != value) dirs[name] = newValue;
         }
       }
@@ skipping 668 matching lines @@
       List<String> args,
       List<String> newArgs,
       Map expect
     }) {
     return new Configuration(
       description: description ?? this.description,
       root: root ?? this.root,
       config: config ?? this.config,
       mainFile: mainFile ??
           ((main == null) ? this.mainFile : "${this.mainPath}$main.dart"),
-      args: args ?? ([]..addAll(newArgs ?? const [])..addAll(this.args)),
+      args:
+          args ?? (<String>[]..addAll(newArgs ?? const <String>[])
+                             ..addAll(this.args)),
       expect: expect == null
           ? this.expect
           : new Map.from(this.expect)..addAll(expect ?? const {}));
   }
 
   // For debugging.
   String toString() {
     return "Configuration($description\n"
       "  root  : $root\n"
       "  config: $config\n"
@@ skipping 16 matching lines @@
 void insertFileAt(Map file, Map http,
                   String path, String name, String content) {
   var parts = path.split('/').toList();
   var dir = (parts[0] == "%file") ? file : http;
   for (var i = 1; i < parts.length - 1; i++) {
     var entry = parts[i];
     dir = dir[entry] ?? (dir[entry] = {});
   }
   dir[name] = content;
 }