Drop support for older Barback versions.

test/build/defaults_to_release_mode_test.dart

Index: test/build/defaults_to_release_mode_test.dart
diff --git a/test/build/defaults_to_release_mode_test.dart b/test/build/defaults_to_release_mode_test.dart
index 2db190d029c64f564a4bda6c6450083d8f514cc8..11199b7cc44336660ef42a06ccbab3f64813b00f 100644
--- a/test/build/defaults_to_release_mode_test.dart
+++ b/test/build/defaults_to_release_mode_test.dart
@@ -28,32 +28,30 @@ class ModeTransformer extends Transformer {
 """;
 
 main() {
-  withBarbackVersions("any", () {
-    integration("defaults to release 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']);
-
-      schedulePub(args: ["build"]);
-
-      d.dir(appPath, [
-        d.dir('build', [
-          d.dir('web', [
-            d.file('foo.out', 'release')
-          ])
-        ])
-      ]).validate();
-    });
-  });
+   integration("defaults to release 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']);
+
+     schedulePub(args: ["build"]);
+
+     d.dir(appPath, [
+       d.dir('build', [
+         d.dir('web', [
+           d.file('foo.out', 'release')
+         ])
+       ])
+     ]).validate();
+   });
 }