Add fourslash test case

tests/cases/fourslash/jsSpecialAssignmentMerging.ts

@@ -0,0 +1,22 @@
+/// <reference path="fourslash.ts" />
+// @noEmit: true
+// @allowJs: true
+// @checkJs: true
+
+// @Filename: b.d.ts
+//// declare class C { }
+// @Filename: a.js
+//// C.prototype = { m: "q"; }
+// @Filename: test.js
+//// var c = new C()
+//// /*1*/
+//// var c = new C()
+
+// #24015
+// This failed with 13 and up on my machine, so 20 is 2**7 more than needed.
+for (let i = 0; i < 20; i++) {
+    goTo.marker('1');
+    edit.insertLine('c');
+
+    verify.getSemanticDiagnostics([])
+}