Improve assertion in computePositionOfLineAndCharacter (microsoft#21361)

Author: Andy

src/compiler/core.ts

@@ -340,6 +340,10 @@ namespace ts {
         return false;
     }
 
+    export function arraysEqual<T>(a: ReadonlyArray<T>, b: ReadonlyArray<T>, equalityComparer: EqualityComparer<T> = equateValues): boolean {
+        return a.length === b.length && a.every((x, i) => equalityComparer(x, b[i]));
+    }
+
     export function indexOfAnyCharCode(text: string, charCodes: ReadonlyArray<number>, start?: number): number {
         for (let i = start || 0; i < text.length; i++) {
             if (contains(charCodes, text.charCodeAt(i))) {

src/compiler/scanner.ts

@@ -335,7 +335,10 @@ namespace ts {
 
     /* @internal */
     export function computePositionOfLineAndCharacter(lineStarts: ReadonlyArray<number>, line: number, character: number, debugText?: string): number {
-        Debug.assert(line >= 0 && line < lineStarts.length);
+        if (line < 0 || line >= lineStarts.length) {
+            Debug.fail(`Bad line number. Line: ${line}, lineStarts.length: ${lineStarts.length} , line map is correct? ${debugText !== undefined ? arraysEqual(lineStarts, computeLineStarts(debugText)) : "unknown"}`);
+        }
+
         const res = lineStarts[line] + character;
         if (line < lineStarts.length - 1) {
             Debug.assert(res < lineStarts[line + 1]);