Bump SDK constraints for pub (#236)

Use a 2.12.0 lower bound since pub does not understand allowed
experiments for earlier versions.

Use a 3.0.0 upper bound to avoid a warning in pub and to give some
flexibility in publishing for stable.

Author: natebosch

CHANGELOG.md

@@ -1,3 +1,8 @@
+## v 2.1.0-nullsafety.5
+
+* Update SDK constraints to `>=2.12.0-0 <3.0.0` based on beta release
+  guidelines.
+
 ## v 2.1.0-nullsafety.4
 
 * Allow prerelease versions of the 2.12 sdk.

pubspec.yaml

@@ -1,11 +1,10 @@
 name: vector_math
-version: 2.1.0-nullsafety.4
+version: 2.1.0-nullsafety.5
 description: A Vector Math library for 2D and 3D applications.
 homepage: https://github.com/google/vector_math.dart
 
 environment:
-  # This must remain a tight constraint until nnbd is stable
-  sdk: '>=2.10.0-0 <2.12.0'
+  sdk: ">=2.12.0-0 <3.0.0"
 
 dev_dependencies:
   benchmark_harness: any