Bring in the latest pub

CHANGELOG.md

 ## 1.23.0
 
 ### Language
 * Allow using URI strings in `part of` declarations to refer to the
   importing library.
 
 ### Core library changes
 
 * `dart:core`
   * Added `Uri.isScheme` function to check the scheme of a URI.
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   * Imports to `dart:io` are allowed, but the imported library is not supported
     and will likely fail on most APIs at runtime. This change was made as a
     stopgap measure to make it easier to write libraries that share code between
     platforms (like package `http`). This might change again when configuration
     specific imports are supported.
 
 * Pub
   * `pub publish`
     * Warns if a package imports a package that's not a dependency from within
       `lib/` or `bin/`, or a package that's not a dev dependency from within
-      `test/` or `tool/`.
+      `benchmark/`, `example/`, `test/` or `tool/`.
     * No longer produces "UID too large" errors on OS X. All packages are now
       uploaded with the user and group names set to "pub".
     * No longer fails with a stack overflow when uploading a package that uses
       Git submodules.
   * `pub get` and `pub upgrade`
     * Produce more informative error messages if they're run directly in a
       package that uses Flutter.
     * Properly unlock SDK and path dependencies if they have a new version
       that's also valid according to the user's pubspec.
 
@@ skipping 1664 matching lines @@
     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.