Drop support for older Barback versions.

test/serve/defaults_to_debug_mode_test.dart

Index: test/serve/defaults_to_debug_mode_test.dart
diff --git a/test/serve/defaults_to_debug_mode_test.dart b/test/serve/defaults_to_debug_mode_test.dart
index bc38fffbd9a3f3b67600207f93a0fff4af1e8470..3cd329d85a605ebc519b73843b296752a7184ac7 100644
--- a/test/serve/defaults_to_debug_mode_test.dart
+++ b/test/serve/defaults_to_debug_mode_test.dart
@@ -29,26 +29,24 @@ class ModeTransformer extends Transformer {
 """;
 
 main() {
-  withBarbackVersions("any", () {
-    integration("defaults to debug mode", () {
-      d.dir(appPath, [
-        d.pubspec({
-          "name": "myapp",
-          "transformers": ["myapp/src/transformer"]
-        }),
-        d.dir("lib", [d.dir("src", [
-          d.file("transformer.dart", TRANSFORMER)
-        ])]),
-        d.dir("web", [
-          d.file("foo.txt", "foo")
-        ])
-      ]).create();
-
-      createLockFile('myapp', pkg: ['barback']);
-
-      pubServe();
-      requestShouldSucceed("foo.out", "debug");
-      endPubServe();
-    });
+  integration("defaults to debug mode", () {
+    d.dir(appPath, [
+      d.pubspec({
+        "name": "myapp",
+        "transformers": ["myapp/src/transformer"]
+      }),
+      d.dir("lib", [d.dir("src", [
+        d.file("transformer.dart", TRANSFORMER)
+      ])]),
+      d.dir("web", [
+        d.file("foo.txt", "foo")
+      ])
+    ]).create();
+
+    createLockFile('myapp', pkg: ['barback']);
+
+    pubServe();
+    requestShouldSucceed("foo.out", "debug");
+    endPubServe();
   });
 }