Bump the SDK constraint on dart_internal to permit Dart 2.8.0.

Change-Id: I8f9dfcd2c361b28f62e7388fc642414efbd120aa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/141021
Reviewed-by: Nicholas Shahan <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>
Auto-Submit: Bob Nystrom <[email protected]>

Author: munificent

pkg/dart_internal/pubspec.yaml

@@ -1,5 +1,5 @@
 name: dart_internal
-version: 0.1.8
+version: 0.1.9
 author: "Dart Team <[email protected]>"
 homepage: http://www.dartlang.org
 description: >
@@ -16,4 +16,4 @@ description: >
 environment:
   # Restrict the upper bound so that we can remove support for this in a later
   # version of the SDK without it being a breaking change.
-  sdk: ">=2.0.0-dev.12.0 <2.8.0"
+  sdk: ">=2.0.0-dev.12.0 <2.9.0"