Drop support for older Barback versions.

test/run/runs_transformer_in_entrypoint_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';
 import '../serve/utils.dart';
 
 const SCRIPT = """
 const TOKEN = "hi";
 main() {
   print(TOKEN);
 }
 """;
 
 main() {
-  withBarbackVersions("any", () {
-    integration('runs transformers in the entrypoint package', () {
-      d.dir(appPath, [
-        d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-        d.dir("lib", [d.dir("src", [
-          d.file("transformer.dart", dartTransformer("transformed"))
-        ])]),
-        d.dir("bin", [
-          d.file("hi.dart", SCRIPT)
-        ])
-      ]).create();
+  integration('runs transformers in the entrypoint package', () {
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp/src/transformer"]
+      }),
+      d.dir("lib", [d.dir("src", [
+        d.file("transformer.dart", dartTransformer("transformed"))
+      ])]),
+      d.dir("bin", [
+        d.file("hi.dart", SCRIPT)
+      ])
+    ]).create();
 
-      createLockFile('myapp', pkg: ['barback']);
+    createLockFile('myapp', pkg: ['barback']);
 
-      var pub = pubRun(args: ["bin/hi"]);
+    var pub = pubRun(args: ["bin/hi"]);
 
-      pub.stdout.expect("(hi, transformed)");
-      pub.shouldExit();
-    });
+    pub.stdout.expect("(hi, transformed)");
+    pub.shouldExit();
   });
 }