Allow a wider pub constraint on barback.

sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart

Index: sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
index c570b62b581f8989a1f0c8c507d09421ff667a04..186ac8e7cb88fed86462366dfd8d9ef48127e403 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/backtracking_solver.dart
@@ -525,17 +525,15 @@ class Traverser {
         // The length check here is to ensure we only add the barback
         // dependency once.
         if (dep.name == "barback" && dependencies.length == 1) {
-          var range = new VersionRange(
-              min: barback.supportedVersion, includeMin: true,
-              max: barback.supportedVersion.nextMinor, includeMax: false);
-          _solver.logSolve('add implicit $range pub dependency on barback');
+          _solver.logSolve('add implicit ${barback.supportedVersions} pub '
+              'dependency on barback');
 
           // Use the same source and description as the explicit dependency.
           // That way, this doesn't fail with a source/desc conflict if users
           // (like Dart team members) use things like a path dependency to
           // find barback.
-          var barbackDep = new PackageDep(dep.name, dep.source, range,
-              dep.description);
+          var barbackDep = new PackageDep(dep.name, dep.source,
+              barback.supportedVersions, dep.description);
           dependencies.add(new Dependency("pub itself", barbackDep));
         }