Update SDK upper bound to 3.0.0.

Prepare to release.
diff --git a/pubspec.yaml b/pubspec.yaml
index 80b5c27..dccc19b 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,11 +1,11 @@
 name: resource
-version: 2.1.5-dev
+version: 2.1.5
 description: Reading resource data from (package and other) files.
 author: Dart Team <misc@dartlang.org>
 homepage: https://github.com/dart-lang/resource
 
 environment:
-  sdk: '>=2.0.0-dev.61.0 <2.0.0'
+  sdk: '>=2.0.0-dev.61.0 <3.0.0'
 
 dependencies:
   typed_data: "^1.0.0"