Fix SDK constraints in analysis_server, front_end, and kernel.

Yesterday I bumped the SDK constraints for these packages to 3.12.0
(see https://dart-review.googlesource.com/c/sdk/+/487521 and
https://dart-review.googlesource.com/c/sdk/+/487542). Today I realized
that this makes a pub solve impossible, since Dart version 3.12.0
hasn't been published yet. That in turn prevents unit tests from being
run in the VSCode IDE.

Changing the SDK constraints to `3.12.0-0` fixes the problem, allowing
the tests to be run under any unreleased version of 3.12.0.

Change-Id: Ia018558dafae9dc958b2b39d19ec1e1d6a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/487880
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server/pubspec.yaml

@@ -3,7 +3,7 @@ name: analysis_server
 publish_to: none
 
 environment:
-  sdk: '^3.12.0'
+  sdk: '^3.12.0-0'
 
 resolution: workspace
 

pkg/front_end/pubspec.yaml

@@ -6,7 +6,7 @@ name: front_end
 publish_to: none
 
 environment:
-  sdk: '^3.12.0'
+  sdk: '^3.12.0-0'
 
 workspace:
   - testcases

pkg/kernel/pubspec.yaml

@@ -6,7 +6,7 @@ name: kernel
 publish_to: none
 
 environment:
-  sdk: '^3.12.0'
+  sdk: '^3.12.0-0'
 
 resolution: workspace