Bring in the latest pub.

CHANGELOG.md

 ## 1.19.0
 
 ### Language changes
 
 * The language now allows a trailing comma after the last argument of a call and
  the last parameter of a function declaration. This can make long argument or
  parameter lists easier to maintain, as commas can be left as-is when
  reordering lines. For details, see SDK issue
  [26644](https://github.com/dart-lang/sdk/issues/26644).
 
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         dependencies:
           flutter_driver:
             sdk: flutter
             version: ^0.0.1
 
     A Flutter `sdk` dependency will only be satisfiable when pub is running in
     the context of the `flutter` executable, and when the Flutter SDK contains a
     package with the given name whose version matches the constraint.
 
+  * Added a `--no-packages-dir` flag to `pub get`, `pub upgrade`, and `pub
+    downgrade`. When this flag is passed, pub will not generate a `packages/`
+    directory, and will remove that directory and any symlinks to it if they
+    exist. Note that this replaces the unsupported `--no-package-symlinks` flag.
+
   * `tar` files on Linux are now created with `0` as the user and group IDs.
     This fixes a crash when publishing packages while using Active Directory.
 
   * Fixed a bug where packages from a hosted HTTP URL were considered the same
     as packages from an otherwise-identical HTTPS URL.
 
   * Fixed timer formatting for timers that lasted longer than a minute.
 
   * Eliminate some false negatives when determining whether global executables
     are on the user's executable path.
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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.