Revert "allow the 2.9.x stable sdk in the meta null_safety prerelease"

This reverts commit 0a2f674.

Reason for revert: This accidentally allows 2.10 dev releases as well

Original change's description:
> allow the 2.9.x stable sdk in the meta null_safety prerelease
> 
> Change-Id: I383e630cf4edba19e4b8ee38adfb0c276eff1d33
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154750
> Auto-Submit: Jake Macdonald <[email protected]>
> Commit-Queue: Jake Macdonald <[email protected]>
> Commit-Queue: Nate Bosch <[email protected]>
> Reviewed-by: Nate Bosch <[email protected]>

[email protected],[email protected]

Change-Id: I3461eeda4155ca40e4a80cc492b61d10d6c83646
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/154820
Reviewed-by: Jake Macdonald <[email protected]>
Commit-Queue: Jake Macdonald <[email protected]>

Author: jakemac53

pkg/meta/CHANGELOG.md

@@ -1,7 +1,3 @@
-## 1.3.0-nullsafety.1
-
-* Allow the 2.9.x stable sdk.
-
 ## 1.3.0-nullsafety
 
 * Opt into null safety.

pkg/meta/pubspec.yaml

@@ -1,11 +1,12 @@
 name: meta
-version: 1.3.0-nullsafety.1
+version: 1.3.0-nullsafety
 homepage: https://github.com/dart-lang/sdk/tree/master/pkg/meta
 description: >
  This library contains the declarations of annotations that developers can use
  to express the intentions that otherwise can't be deduced by statically
  analyzing the source code. These annotations are intended to be used by tools
  to provide a better user experience.
 environment:
-  # This must remain a tight upper bound until nnbd is stable.
-  sdk: '>=2.9.0-18.0 <2.10.0'
+  # This must remain a tight constraint (only allow dev versions) until nnbd is
+  # stable.
+  sdk: '>=2.9.0-18.0 <2.9.0'