Bring in the latest pub.

CHANGELOG.md

 ## 1.14.0
 
 ### Core library changes
 * `dart:convert`
   * `Base64Decoder.convert` now takes optional `start` and `end` parameters.
 
 * `dart:core`
   * Added `current` getter to `StackTrace` class.
   * Added `Uri.data` getter for `data:` URIs, and `UriData` class for the
     return type.
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   * Other optimizations and bug fixes.
 
 * Pub
 
   * **Breaking:** Pub now eagerly emits an error when a pubspec's "name" field
     is not a valid Dart identifier. Since packages with non-identifier names
     were never allowed to be published, and some of them already caused crashes
     when being written to a `.packages` file, this is unlikely to break many
     people in practice.
 
+  * **Breaking:** Support for `barback` versions prior to 0.15.0 (released July
+    2014) has been dropped. Pub will no longer install these older barback
+    versions.
+
   * `pub serve` now GZIPs the assets it serves to make load times more similar
     to real-world use-cases.
 
   * `pub deps` now supports a `--no-dev` flag, which causes it to emit the
     dependency tree as it would be if no `dev_dependencies` were in use. This
     makes it easier to see your package's dependency footprint as your users
     will experience it.
 
   * `pub global run` now detects when a global executable's SDK constraint is no
     longer met and errors out, rather than trying to run the executable anyway.
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     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.