allow the 2.12 prerelease sdks (dart-lang/string_scanner#26)

pkgs/string_scanner/CHANGELOG.md

diff --git a/pkgs/string_scanner/CHANGELOG.md b/pkgs/string_scanner/CHANGELOG.md
index 3166551..4379f83 100644
--- a/pkgs/string_scanner/CHANGELOG.md
+++ b/pkgs/string_scanner/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 1.1.0-nullsafety.2
+
+* Allow prerelease versions of the 2.12 sdk.
+
 ## 1.1.0-nullsafety.1
 
 - Allow 2.10 stable and 2.11.0 dev SDK versions.

pkgs/string_scanner/pubspec.yaml

diff --git a/pkgs/string_scanner/pubspec.yaml b/pkgs/string_scanner/pubspec.yaml
index 566f482..9684e14 100644
--- a/pkgs/string_scanner/pubspec.yaml
+++ b/pkgs/string_scanner/pubspec.yaml
@@ -1,12 +1,12 @@
 name: string_scanner
-version: 1.1.0-nullsafety.1
+version: 1.1.0-nullsafety.2
 
 description: A class for parsing strings using a sequence of patterns.
 homepage: https://github.com/dart-lang/string_scanner
 
 environment:
   # This must remain a tight constraint until nnbd is stable
-  sdk: '>=2.10.0-0 <2.11.0'
+  sdk: '>=2.10.0-0 <2.12.0'
 
 dependencies:
   charcode: '>=1.2.0-nullsafety <1.2.0'