Bump analyzer dependency.

Updates `test` to be constraint-compatible with the latest analyzer (0.28.0-alpha.1).

BUG=
R=brianwilkerson@google.com, nweiz@google.com

Review URL: https://codereview.chromium.org//2255083003 .
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2ebbd3e..b76bfc4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.12.15+4
+
+* Widen version constraint on `analyzer`.
+
 ## 0.12.15+3
 
 * Move `nestingMiddleware` to `lib/src/util/path_handler.dart` to enable a
diff --git a/pubspec.yaml b/pubspec.yaml
index 6b47591..ffba7da 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,12 @@
 name: test
-version: 0.12.15+3
+version: 0.12.15+4
 author: Dart Team <misc@dartlang.org>
 description: A library for writing dart unit tests.
 homepage: https://github.com/dart-lang/test
 environment:
   sdk: '>=1.14.0 <2.0.0'
 dependencies:
-  analyzer: '>=0.23.0 <0.28.0'
+  analyzer: '>=0.23.0 <0.29.0'
   args: '^0.13.1'
   async: '^1.8.0'
   barback: '>=0.14.0 <0.16.0'