Improve parallelism when loading transformer plugins.

sdk/lib/_internal/pub/test/test_pub.dart

 // Copyright (c) 2013, 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.
 
 /// Test infrastructure for testing pub. Unlike typical unit tests, most pub
 /// tests are integration tests that stage some stuff on the file system, run
 /// pub, and then validate the results. This library provides an API to build
 /// tests like that.
 library test_pub;
 
@@ skipping 266 matching lines @@
   }, 'initializing the package server');
 }
 
 /// Converts [value] into a YAML string.
 String yaml(value) => JSON.encode(value);
 
 /// The full path to the created sandbox directory for an integration test.
 String get sandboxDir => _sandboxDir;
 String _sandboxDir;
 
+/// The path to the Dart repo's packages.
+final String pkgPath = path.absolute(path.join(
+    path.dirname(Platform.executable),
+    '..', '..', '..', '..', 'pkg'));
+
 /// The path of the package cache directory used for tests. Relative to the
 /// sandbox directory.
 final String cachePath = "cache";
 
 /// The path of the mock app directory used for tests. Relative to the sandbox
 /// directory.
 final String appPath = "myapp";
 
 /// The path of the packages directory in the mock app used for tests. Relative
 /// to the sandbox directory.
@@ skipping 370 matching lines @@
     Iterable<String> pkg, Map<String, String> hosted}) {
   var dependencies = {};
 
   if (sandbox != null) {
     for (var package in sandbox) {
       dependencies[package] = '../$package';
     }
   }
 
   if (pkg != null) {
-    var pkgDir = path.absolute(path.join(
-        path.dirname(Platform.executable),
-        '..', '..', '..', '..', 'pkg'));
-
     _addPackage(String package) {
       if (dependencies.containsKey(package)) return;
 
       var packagePath;
       if (package == 'barback') {
         if (_barbackDir == null) {
           throw new StateError("createLockFile() can only create a lock file "
               "with a barback dependency within a withBarbackVersions() "
               "block.");
         }
         packagePath = _barbackDir;
       } else {
-        packagePath = path.join(pkgDir, package);
+        packagePath = path.join(pkgPath, package);
       }
 
       dependencies[package] = packagePath;
       var pubspec = loadYaml(
           readTextFile(path.join(packagePath, 'pubspec.yaml')));
       var packageDeps = pubspec['dependencies'];
       if (packageDeps == null) return;
       packageDeps.keys.forEach(_addPackage);
     }
 
@@ skipping 204 matching lines @@
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     return description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }
 
 /// A [StreamMatcher] that matches multiple lines of output.
 StreamMatcher emitsLines(String output) => inOrder(output.split("\n"));