Lower dependency on dev.1 so that we can already start testing.

Review URL: https://codereview.chromium.org//2439633003 .
diff --git a/pubspec.yaml b/pubspec.yaml
index f78a75a..f92b73d 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,9 +1,9 @@
 name: collection
-version: 1.10.0
+version: 1.10.1
 author: Dart Team <misc@dartlang.org>
 description: Collections and utilities functions and classes related to collections.
 homepage: https://www.github.com/dart-lang/collection
 environment:
-  sdk: '>=1.21.0 <2.0.0'
+  sdk: '>=1.21.0-dev.1.0 <2.0.0'
 dev_dependencies:
   test: '^0.12.0'