Language tests updated to use const instead of final.

tests/language/library12.dart

Index: tests/language/library12.dart
diff --git a/tests/language/library12.dart b/tests/language/library12.dart
index 379b96a2edaba059d17be6fcd587527c0a8649e6..635a8f2f5e5f2843141a91ead350eb149aa91970 100644
--- a/tests/language/library12.dart
+++ b/tests/language/library12.dart
@@ -37,7 +37,7 @@ interface Library12Interface {
   Library12 addObjects(Library12 value1, Library12 value2);
 }
 
-final int top_level12 = 10;
+const int top_level12 = 10;
 top_level_func12() {
   return 20;
 }