Improve parallelism when loading transformer plugins.

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

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index 2380ed31d10e97c22177401a1f972b7de5a88863..f99305fa1a47545fff19f3a798f26275a64dd249 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -284,6 +284,11 @@ String yaml(value) => JSON.encode(value);
 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";
@@ -674,10 +679,6 @@ void createLockFile(String package, {Iterable<String> sandbox,
   }
 
   if (pkg != null) {
-    var pkgDir = path.absolute(path.join(
-        path.dirname(Platform.executable),
-        '..', '..', '..', '..', 'pkg'));
-
     _addPackage(String package) {
       if (dependencies.containsKey(package)) return;
 
@@ -690,7 +691,7 @@ void createLockFile(String package, {Iterable<String> sandbox,
         }
         packagePath = _barbackDir;
       } else {
-        packagePath = path.join(pkgDir, package);
+        packagePath = path.join(pkgPath, package);
       }
 
       dependencies[package] = packagePath;