Prepare to publish for stable null safety (#25)

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ec88d5e..714c392 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 2.1.0
+
+* Stable release for null safety.
+
 ## 2.1.0-nullsafety.4
 
 * Update SDK constraints to `>=2.12.0-0 <3.0.0` based on beta release

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 4b10b1b..b4de173 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: source_map_stack_trace
-version: 2.1.0-nullsafety.4
+version: 2.1.0
 description: A package for applying source maps to stack traces.
 homepage: https://github.com/dart-lang/source_map_stack_trace
 
@@ -7,9 +7,9 @@
   sdk: ">=2.12.0-0 <3.0.0"
 
 dependencies:
-  path: '>=1.8.0-nullsafety <1.8.0'
-  stack_trace: '>=1.10.0-nullsafety <1.10.0'
-  source_maps: '>=0.10.10-nullsafety <0.10.10'
+  path: ^1.8.0
+  stack_trace: ^1.10.0
+  source_maps: ^0.10.10
 
 dev_dependencies:
   source_span: ^1.8.0-nullsafety