Allow a wider pub constraint on barback.

sdk/lib/_internal/pub/test/implicit_barback_dependency_test.dart

 // Copyright (c) 2013, 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 '../lib/src/barback.dart' as barback;
 import '../lib/src/version.dart';
 
 main() {
   initConfig();
 
-  var current = barback.supportedVersion.toString();
-  var previous = new Version(barback.supportedVersion.major,
-      barback.supportedVersion.minor - 1, 0).toString();
-  var nextPatch = barback.supportedVersion.nextPatch.toString();
-  var max = barback.supportedVersion.nextMinor.toString();
+  var current = barback.supportedVersions.min.toString();
+  var previous = new Version(barback.supportedVersions.min.major,
+      barback.supportedVersions.min.minor - 1, 0).toString();
+  var nextPatch = barback.supportedVersions.min.nextPatch.toString();
+  var max = barback.supportedVersions.max.toString();
 
   forBothPubGetAndUpgrade((command) {
     integration("implicitly constrains barback to versions pub supports", () {
       servePackages([
         packageMap("barback", previous),
         packageMap("barback", current),
         packageMap("barback", nextPatch),
         packageMap("barback", max)
       ]);
 
       d.appDir({
         "barback": "any"
       }).create();
 
       pubCommand(command);
 
       d.packagesDir({
-        "barback": barback.supportedVersion.nextPatch.toString()
+        "barback": barback.supportedVersions.min.nextPatch.toString()
       }).validate();
     });
 
     integration("discovers transitive dependency on barback", () {
       servePackages([
         packageMap("barback", previous),
         packageMap("barback", current),
         packageMap("barback", nextPatch),
         packageMap("barback", max)
       ]);
@@ skipping 80 matching lines @@
     ]);
 
     d.appDir({"barback": previous}).create();
 
     pubGet(error: """
 Incompatible version constraints on barback:
 - myapp depends on version $previous
 - pub itself depends on version >=$current <$max""");
   });
 }