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

BUG=#27399
BUG=dart-lang/pub#1450
R=kustermann@google.com

Review URL: https://codereview.chromium.org/2374143005 .

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

DEPS

@@ -91,7 +91,7 @@ vars = {
   "pool_tag": "@1.2.4",
   "protobuf_tag": "@0.5.3",
   "pub_cache_tag": "@v0.1.0",
-  "pub_rev": "@4ef3e3e8ad8089733d617505cc66fa3d8049b4ae",
+  "pub_rev": "@3dd04bd17ba269ccdd34502a253041dd96ded3be",
   "pub_semver_tag": "@1.3.0",
   "quiver_tag": "@0.22.0",
   "resource_rev":"@a49101ba2deb29c728acba6fb86000a8f730f4b1",