dispose snapshot only if new file differs from the old file

Author: vladima

src/services/services.ts

@@ -1878,7 +1878,7 @@ module ts {
                     newSourceFile.nameTable = undefined;
 
                     // dispose all resources held by old script snapshot
-                    if (sourceFile.scriptSnapshot) {
+                    if (sourceFile !== newSourceFile && sourceFile.scriptSnapshot) {
                         if (sourceFile.scriptSnapshot.dispose) {
                             sourceFile.scriptSnapshot.dispose();
                         }