Drop support for older Barback versions.

test/transformer/runs_a_transformer_group_test.dart

Index: test/transformer/runs_a_transformer_group_test.dart
diff --git a/test/transformer/runs_a_transformer_group_test.dart b/test/transformer/runs_a_transformer_group_test.dart
index 3839c475765a399616248a958e0a7961bdd19dc1..854ced9f388ef599ec98b8a9fdf8fbc3f937407f 100644
--- a/test/transformer/runs_a_transformer_group_test.dart
+++ b/test/transformer/runs_a_transformer_group_test.dart
@@ -21,27 +21,25 @@ class RewriteGroup implements TransformerGroup {
 """;
 
 main() {
-  withBarbackVersions("any", () {
-    integration("runs a transformer group", () {
-      d.dir(appPath, [
-        d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/group"]
-        }),
-        d.dir("lib", [d.dir("src", [
-          d.file("transformer.dart", REWRITE_TRANSFORMER),
-          d.file("group.dart", GROUP)
-        ])]),
-        d.dir("web", [
-          d.file("foo.txt", "foo")
-        ])
-      ]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe();
-      requestShouldSucceed("foo.out", "foo.out");
-      endPubServe();
-    });
+  integration("runs a transformer group", () {
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp/src/group"]
+      }),
+      d.dir("lib", [d.dir("src", [
+        d.file("transformer.dart", REWRITE_TRANSFORMER),
+        d.file("group.dart", GROUP)
+      ])]),
+      d.dir("web", [
+        d.file("foo.txt", "foo")
+      ])
+    ]).create();
+
+    createLockFile('myapp', pkg: ['barback']);
+
+    pubServe();
+    requestShouldSucceed("foo.out", "foo.out");
+    endPubServe();
   });
 }