Bring in the latest pub.

CHANGELOG.md

Index: CHANGELOG.md
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+## 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