Merge pull request #24271 from Microsoft/portRegExpDotAllFix

Port changes in #24238 to source file

src/lib/es2018.regexp.d.ts

@@ -8,4 +8,12 @@ interface RegExpExecArray {
     groups?: {
         [key: string]: string
     }
+}
+
+interface RegExp {
+    /**
+     * Returns a Boolean value indicating the state of the dotAll flag (s) used with a regular expression.
+     * Default is false. Read-only.
+     */
+    readonly dotAll: boolean;
 }