Release protobuf-3.0.0, protoc_plugin-21.0.0 (#843)

diff --git a/protobuf/CHANGELOG.md b/protobuf/CHANGELOG.md
index d8d3b83..e5ef267 100644
--- a/protobuf/CHANGELOG.md
+++ b/protobuf/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 3.0.0-dev
+## 3.0.0
 
 * Require Dart `2.19`.
 * Remove `ReadonlyMessageMixin` ([#183], [#644])
diff --git a/protobuf/pubspec.yaml b/protobuf/pubspec.yaml
index 7489143..696e794 100644
--- a/protobuf/pubspec.yaml
+++ b/protobuf/pubspec.yaml
@@ -1,12 +1,12 @@
 name: protobuf
-version: 3.0.0-dev
+version: 3.0.0
 description: >-
   Runtime library for protocol buffers support. Use with package:protoc_plugin
   to generate dart code for your '.proto' files.
 repository: https://github.com/google/protobuf.dart/tree/master/protobuf
 
 environment:
-  sdk: '>=2.19.0 <3.0.0'
+  sdk: '>=2.19.0 <4.0.0'
 
 dependencies:
   collection: ^1.15.0
diff --git a/protoc_plugin/CHANGELOG.md b/protoc_plugin/CHANGELOG.md
index fd2ad42..bedab39 100644
--- a/protoc_plugin/CHANGELOG.md
+++ b/protoc_plugin/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 21.0.0-dev
+## 21.0.0
 
 * Identifiers `fromBuffer`, `fromJson`, `$_defaultFor`, `initByValue` are no
   longer reserved. Proto fields with those Dart names will no longer have a
diff --git a/protoc_plugin/pubspec.yaml b/protoc_plugin/pubspec.yaml
index 9d41023..2b48e84 100644
--- a/protoc_plugin/pubspec.yaml
+++ b/protoc_plugin/pubspec.yaml
@@ -1,15 +1,15 @@
 name: protoc_plugin
-version: 21.0.0-dev
+version: 21.0.0
 description: A protobuf protoc compiler plugin used to generate Dart code.
 repository: https://github.com/google/protobuf.dart/tree/master/protoc_plugin
 
 environment:
-  sdk: '>=2.19.0 <3.0.0'
+  sdk: '>=2.19.0 <4.0.0'
 
 dependencies:
   fixnum: ^1.0.0
   path: ^1.8.0
-  protobuf: ^2.0.0
+  protobuf: ^3.0.0
 
 dev_dependencies:
   collection: ^1.15.0