Bring in the latest pub.

CHANGELOG.md

 ## 1.22.0
 
 ### Language
 
   * Breaking change: ['Generalized tear-offs'](https://github.com/gbracha/generalizedTearOffs/blob/master/proposal.md)
     are no longer supported, and will cause errors. We updated the language spec
     and added warnings in 1.21, and are now taking the last step to fully
     de-support them. They were previously supported in the VM only.
 
   * The `assert()` statement has been expanded to support an optional second
     `message` argument (SDK issue [27342](https://github.com/dart-lang/sdk/issues/27342)).
 
     The message is displayed if the assert fails. It can be any object, and it
     is accessible as `AssertionError.message`. It can be used to provide more
     user friendly exception outputs. As an example, the following assert:
 
     ```dart
     assert(configFile != null, "Tool config missing. Please see https://goo.gl/k8iAi for details.");
     ```
 
     would produce the following exception output:
 
     ```
     Unhandled exception:
-    'file:///Users/mit/tmp/tool/bin/main.dart': Failed assertion: line 9 pos 10: 
+    'file:///Users/mit/tmp/tool/bin/main.dart': Failed assertion: line 9 pos 10:
     'configFile != null': Tool config missing. Please see https://goo.gl/k8iAi for details.
     #0      _AssertionError._doThrowNew (dart:core-patch/errors_patch.dart:33)
     #1      _AssertionError._throwNew (dart:core-patch/errors_patch.dart:29)
     #2      main (file:///Users/mit/tmp/tool/bin/main.dart:9:10)
     ```
 
 ### Tool changes
 
 * Dart2Js
 
   * Remove support for (long-time deprecated) mixin typedefs.
 
 * Pub
 
   * Avoid using a barback asset server for executables unless they actually use
     transformers. This makes precompilation substantially faster, produces
     better error messages when precompilation fails, and allows
     globally-activated executables to consistently use the
     `Isolate.resolvePackageUri()` API.
 
-  * On POSIX systems, always ignore packages' original file owners and
+  * On Linux systems, always ignore packages' original file owners and
     permissions when extracting those packages. This was already the default
     under most circumstances.
 
   * Properly close the standard input stream of child processes started using
     `pub run`.
 
   * Handle parse errors from the package cache more gracefully. A package whose
     pubspec can't be parsed will now be ignored by `pub get --offline` and
     deleted by `pub cache repair`.
 
+  * Make `pub run` run executables in spawned isolates. This lets them handle
+    signals and use standard IO reliably.
 
 ## 1.21.0 - 2016-12-07
 
 ### Language
 
 * Support generic method syntax. Type arguments are not available at
   runtime. For details, check the
   [informal specification](https://gist.github.com/eernstg/4353d7b4f669745bed3a5423e04a453c).
 * Support access to initializing formals, e.g., the use of `x` to initialize
  `y` in `class C { var x, y; C(this.x): y = x; }`.
@@ skipping 1395 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.