Drop support for older Barback versions.

test/run/runs_a_generated_script_test.dart

 // Copyright (c) 2014, 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 '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 const TRANSFORMER = """
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 
 class DartTransformer extends Transformer {
   DartTransformer.asPlugin();
 
   String get allowedExtensions => '.in';
 
   void apply(Transform transform) {
     transform.addOutput(new Asset.fromString(
         new AssetId("myapp", "bin/script.dart"),
         "void main() => print('generated');"));
   }
 }
 """;
 
 main() {
-  withBarbackVersions("any", () {
-    integration('runs a script generated from scratch by a transformer', () {
-      d.dir(appPath, [
-        d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-        d.dir("lib", [d.dir("src", [
-          d.file("transformer.dart", TRANSFORMER),
-          d.file("primary.in", "")
-        ])])
-      ]).create();
+  integration('runs a script generated from scratch by a transformer', () {
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp/src/transformer"]
+      }),
+      d.dir("lib", [d.dir("src", [
+        d.file("transformer.dart", TRANSFORMER),
+        d.file("primary.in", "")
+      ])])
+    ]).create();
 
-      createLockFile('myapp', pkg: ['barback']);
+    createLockFile('myapp', pkg: ['barback']);
 
-      var pub = pubRun(args: ["bin/script"]);
+    var pub = pubRun(args: ["bin/script"]);
 
-      pub.stdout.expect("generated");
-      pub.shouldExit();
-    });
+    pub.stdout.expect("generated");
+    pub.shouldExit();
   });
 }