add cache breaker to make sure we can go back easily from this change…

…, ex. revert

packages/jest-runtime/src/script_transformer.js

@@ -44,6 +44,9 @@ const configToJsonMap = new Map();
 // Cache regular expressions to test whether the file needs to be preprocessed
 const ignoreCache: WeakMap<ProjectConfig, ?RegExp> = new WeakMap();
 
+// To reset the cache for specific changesets (rather than package version).
+const CACHE_BREAKER = '1';
+
 class ScriptTransformer {
   static EVAL_RESULT_VARIABLE: string;
   _config: ProjectConfig;
@@ -79,6 +82,7 @@ class ScriptTransformer {
         .update(configString)
         .update(instrument ? 'instrument' : '')
         .update(mapCoverage ? 'mapCoverage' : '')
+        .update(CACHE_BREAKER)
         .digest('hex');
     }
   }