Allow pub get/upgrade to run on a number of packages in pkg/

Zero runtime affect to these unpublished packages
Allows using standard tools such as dart test, etc
Also allows better checking of dependency constraints

TEST=no effect on SDK build process

Change-Id: Ic6fbe6ce8f57724a87c058eccad64ce8e7473af1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/218180
Auto-Submit: Kevin Moore <kevmoo@google.com>
Reviewed-by: Devon Carew <devoncarew@google.com>

pkg/expect/pubspec.yaml

@@ -12,5 +12,8 @@ environment:
   sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
-  meta:
-    path: ../meta
+  meta: any
+
+dependency_overrides:
+ meta:
+  path: ../meta