Fix analyzer warning in a unit test.

Add initializer for a final field.

[email protected]

BUG=

Review URL: https://codereview.chromium.org/1494233003 .

Author: fsc8000

tests/language/vm/type_propagation_test.dart

@@ -12,7 +12,7 @@ class B {
 }
 
 class C {
-  final f0;
+  final f0 = null;
 
   final a;
   C() : a = new B(0);