Add assertion when classification has unexpected length

Author: Ben Lichtman

src/services/classifier.ts

@@ -499,8 +499,10 @@ namespace ts {
         return { spans, endOfLineState: EndOfLineState.None };
 
         function pushClassification(start: number, end: number, type: ClassificationType): void {
+            const length = end - start;
+            Debug.assert(length >= 0, `Classification had non-positive length of ${length}`);
             spans.push(start);
-            spans.push(end - start);
+            spans.push(length);
             spans.push(type);
         }
     }