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

CHANGELOG.md

 ## 1.20.0
 
 ### Dart VM
 
 * We have improved the way that the VM locates the native code library for a
   native extension (e.g. `dart-ext:` import). We have updated this
   [article on native extensions](https://www.dartlang.org/articles/dart-vm/native-extensions)
   to reflect the VM's improved behavior.
 
 * Linux builds of the VM will now use the `tcmalloc` library for memory
@@ skipping 100 matching lines @@
     class C extends A {}
     ```
 
 ### Tool changes
 
 * `dartfmt` - upgraded to v0.2.10
     * Don't crash on annotations before parameters with trailing commas.
     * 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
 
 * The language now allows a trailing comma after the last argument of a call and
  the last parameter of a function declaration. This can make long argument or
  parameter lists easier to maintain, as commas can be left as-is when
  reordering lines. For details, see SDK issue
  [26644](https://github.com/dart-lang/sdk/issues/26644).
 
@@ skipping 1185 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.