Language tests updated to use const instead of final.

tests/language/library_prefixes_test2.dart

Index: tests/language/library_prefixes_test2.dart
diff --git a/tests/language/library_prefixes_test2.dart b/tests/language/library_prefixes_test2.dart
index 5c2d3128dae0a8b17b9c54e03971095030adc561..7e5e600bb7cfd4ccce1b59e819ece473b4520d9d 100644
--- a/tests/language/library_prefixes_test2.dart
+++ b/tests/language/library_prefixes_test2.dart
@@ -3,12 +3,12 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class Constants {
-  static final PI = 3.14;
-  static final foo = 2;
+  static const PI = 3.14;
+  static const foo = 2;
 }
 
 class A {
-  static final y = 0;
+  static const y = 0;
   int x;
   A() : x = 2 {}
   A.named() : x = 4 {}