Move internal modules to “internal” folder.

.internal/toSource.js

@@ -0,0 +1,23 @@
+/** Used to resolve the decompiled source of functions. */
+const funcToString = Function.prototype.toString;
+
+/**
+ * Converts `func` to its source code.
+ *
+ * @private
+ * @param {Function} func The function to convert.
+ * @returns {string} Returns the source code.
+ */
+function toSource(func) {
+  if (func != null) {
+    try {
+      return funcToString.call(func);
+    } catch (e) {}
+    try {
+      return `${ func }`;
+    } catch (e) {}
+  }
+  return '';
+}
+
+export default toSource;