addressed PR feedback: added comments

Author: vladima

src/services/shims.ts

@@ -248,6 +248,8 @@ namespace ts {
         }
 
         public dispose(): void {
+            // if scriptSnapshotShim is a COM object then property check becomes method call with no arguments
+            // 'in' does not have this effect
             if ("dispose" in this.scriptSnapshotShim) {
                 this.scriptSnapshotShim.dispose();
             }