Bring in the latest pub.

CHANGELOG.md

+## 1.19.0
+
+### Tool Changes
+
+* Pub
+  * Added the ability for packages to declare a constraint on the [Flutter][]
+    SDK:
+
+        environment:
+          flutter: ^0.1.2
+          sdk: >=1.19.0 <2.0.0
+
+    A Flutter constraint will only be satisfiable when pub is running in the
+    context of the `flutter` executable, and when the Flutter SDK version
+    matches the constraint.
+
+  * Added `sdk` as a new package source that fetches packages from a hard-coded
+    SDK. Currently only the `flutter` SDK is supported:
+
+        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.
+
+  * 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.
+
+[Flutter]: https://flutter.io/
+
 ## 1.18.0
 
 ### Core library changes
 
 * `dart:io`
   * Adds file locking modes `FileLock.BLOCKING_SHARED` and
     `FileLock.BLOCKING_EXCLUSIVE`.
 
 ## 1.17.1 - 2016-06-10
 
@@ skipping 1023 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.