Bump pub to the latest version.

CHANGELOG.md

 ## 1.13.0
 
 * `dart:core`
   * `Uri` added `removeFragment` method.
   * `String.allMatches` (implementing `Pattern.allMatches`) is now lazy,
     as all `allMatches` implementations are intended to be.
 
 * `dart:io`
   * `HttpClient` no longer sends URI fragments in the requeust. This is not
     allowed by the HTTP protocol.
     The `HttpServer` still gracefully receives fragments, but discards them
     before delivering the request.
 
 * `dart:async`
   * `StreamTransformer`s created with `fromHandlers` with no `handleError`
     callback now forward stack traces along with errors to the resulting
     streams.
 
+## 1.12.1
+
+### Tool changes
+
+* Pub
+
+  * Pub will now respect `.gitignore` when validating a package before it's
+    published. For example, if a `LICENSE` file exists but is ignored, that is
+    now an error.
+
+  * If the package is in a subdirectory of a Git repository and the entire
+    subdirectory is ignored with `.gitignore`, pub will act as though nothing
+    was ignored instead of uploading an empty package.
+
+  * The heuristics for determining when `pub get` needs to be run before various
+    commands have been improved. There should no longer be false positives when
+    non-dependency sections of the pubspec have been modified.
+
 ## 1.12.0
 
 ### Language changes
 
 * Null-aware operators
     * `??`: if null operator. `expr1 ?? expr2` evaluates to `expr1` if not `null`, otherwise `expr2`.
     * `??=`: null-aware assignment. `v ??= expr` causes `v` to be assigned `expr` only if `v` is `null`.
     * `x?.p`: null-aware access. `x?.p` evaluates to `x.p` if `x` is not `null`, otherwise evaluates to `null`.
     * `x?.m()`: null-aware method invocation. `x?.m()` invokes `m` only if `x` is not `null`.
 
@@ skipping 510 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.