[ddc] Bump minimum sdk version

Preparation to begin migrating dev_compiler libraries to null safety.
With this change any file we remove the old language version comment
from will be opted into null safety.

Change-Id: I567d139a6c6bfa0b8f820b16e3eb87a2f49228b3
Issue: https://github.com/dart-lang/sdk/issues/46617
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206961
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

.dart_tool/package_config.json

@@ -250,7 +250,7 @@
       "name": "dev_compiler",
       "rootUri": "../pkg/dev_compiler",
       "packageUri": "lib/",
-      "languageVersion": "2.3"
+      "languageVersion": "2.12"
     },
     {
       "name": "devtools_shared",
@@ -806,4 +806,4 @@
       "languageVersion": "2.12"
     }
   ]
-}
+}

pkg/dev_compiler/pubspec.yaml

@@ -3,7 +3,7 @@ name: dev_compiler
 publish_to: none
 
 environment:
-  sdk: '>=2.3.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   _fe_analyzer_shared: