Add test with a referenced dts file

Author: Andy Hanson

tests/cases/compiler/commonSourceDirectory_dts.ts

@@ -0,0 +1,22 @@
+// Test that importing a file from `node_modules` does not affect calculation of the common source directory.
+// @noImplicitReferences: true
+// @moduleResolution: node
+// @fullEmitPaths: true
+
+// @filename: /app/lib/bar.d.ts
+declare const y: number;
+
+// @filename: /app/src/index.ts
+/// <reference path="../lib/bar.d.ts" />
+export const x = y;
+
+// @filename: /app/tsconfig.json
+{
+    "compilerOptions": {
+        "outDir": "bin",
+        "sourceMap": true,
+        "mapRoot": "myMapRoot",
+        "sourceRoot": "mySourceRoot",
+        "declaration": true
+    }
+}