Widen the SDK constraint on dart_internal to include 2.7.0.

Change-Id: I55e9b1224f69268ccfb8ba345d3a9d62d3d93838
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127449
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

Author: munificent

pkg/dart_internal/pubspec.yaml

@@ -1,5 +1,5 @@
 name: dart_internal
-version: 0.1.7
+version: 0.1.8
 author: "Dart Team <misc@dartlang.org>"
 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 <2.7.0"
+  sdk: ">=2.0.0-dev.12.0 <2.8.0"