normalize dependency versions

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index dc2dfe5..4a408dd 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 repository: https://github.com/dart-lang/source_map_stack_trace
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   path: ^1.8.0
@@ -12,6 +12,6 @@
   source_maps: ^0.10.10
 
 dev_dependencies:
-  source_span: ^1.8.0-nullsafety
-  test: ^1.16.0-nullsafety
-  pedantic: ^1.10.0-nullsafety
+  source_span: ^1.8.0
+  test: ^1.16.0
+  pedantic: ^1.10.0