Update pub, making --packages-dir false by default

CHANGELOG.md

Index: CHANGELOG.md
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6fdacec45d75f027abd6bc1daadd67e36484f350..7a3828851211b1d423261a4b5e40c7969ef79583 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -118,6 +118,12 @@
     * Always split enum declarations if they end in a trailing comma.
     * Add `--set-exit-if-changed` to set the exit code on a change.
 
+* Pub
+  * Pub no longer generates a `packages/` directory by default.  Instead, it
+    generates a `.packages` file, called a package spec. To generate
+    a `packages/` directory in addition to the package spec, use the
+    `--packages-dir` flag with `pub get`, `pub upgrade`, and `pub downgrade`.
+
 ## 1.19.0
 
 ### Language changes