Drop support for older Barback versions.

lib/src/barback.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.
 
 library pub.barback;
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 
@@ skipping 19 matching lines @@
 /// because most barback features need additional serialization code to be fully
 /// supported in pub, even if they're otherwise backwards-compatible.
 ///
 /// Whenever a new minor or patch version of barback is published, this *must*
 /// be incremented to synchronize with that. See the barback [compatibility
 /// documentation][compat] for details on the relationship between this
 /// constraint and barback's version.
 ///
 /// [compat]: https://gist.github.com/nex3/10942218
 final pubConstraints = {
-  "barback": new VersionConstraint.parse(">=0.13.0 <0.15.3"),
+  "barback": new VersionConstraint.parse(">=0.15.0 <0.15.3"),
   "source_span": new VersionConstraint.parse(">=1.0.0 <2.0.0"),
   "stack_trace": new VersionConstraint.parse(">=0.9.1 <2.0.0")
 };
 
 /// Converts [id] to a "package:" URI.
 ///
 /// This will throw an [ArgumentError] if [id] doesn't represent a library in
 /// `lib/`.
 Uri idToPackageUri(AssetId id) {
   if (!id.path.startsWith('lib/')) {
@@ skipping 29 matching lines @@
   if (parts.length <= index + 1) {
     throw new FormatException(
         'Invalid URL path "${url.path}". Expected package name '
         'after "packages".');
   }
 
   var package = parts[index + 1];
   var assetPath = p.url.join("lib", p.url.joinAll(parts.skip(index + 2)));
   return new AssetId(package, assetPath);
 }