Bump Flutter SDK requirement (#456)

* Update Flutter SDK requirement

* Not a breaking change

* Update CHANGELOG.md

* Update pubspec.yaml

packages/cloud_firestore/CHANGELOG.md

diff --git a/packages/cloud_firestore/CHANGELOG.md b/packages/cloud_firestore/CHANGELOG.md
index bb3ee95..c64355c 100644
--- a/packages/cloud_firestore/CHANGELOG.md
+++ b/packages/cloud_firestore/CHANGELOG.md
@@ -1,3 +1,10 @@
+## 0.4.0
+
+* **Breaking change**. Hide Firestore codec class from public API.
+* Adjusted Flutter SDK constraint to match Flutter release with extensible
+  platform message codec, required already by version 0.3.1.
+* Move each class into separate files
+
 ## 0.3.2
 
 * Support for batched writes.

packages/cloud_firestore/pubspec.yaml

diff --git a/packages/cloud_firestore/pubspec.yaml b/packages/cloud_firestore/pubspec.yaml
index 80b7c92..1124a23 100755
--- a/packages/cloud_firestore/pubspec.yaml
+++ b/packages/cloud_firestore/pubspec.yaml
@@ -3,7 +3,7 @@
   live synchronization and offline support on Android and iOS.
 author: Flutter Team <flutter-dev@googlegroups.com>
 homepage: https://github.com/flutter/plugins/tree/master/packages/cloud_firestore
-version: 0.3.2
+version: 0.4.0
 
 flutter:
   plugin:
@@ -23,4 +23,4 @@
 
 environment:
   sdk: ">=2.0.0-dev.28.0 <3.0.0"
-  flutter: ">=0.1.4 <2.0.0"
+  flutter: ">=0.2.4 <2.0.0"