Updated the files.

my-app/node_modules/webpack/lib/util/StackedMap.js

@@ -0,0 +1,166 @@
+/*
+	MIT License http://www.opensource.org/licenses/mit-license.php
+	Author Tobias Koppers @sokra
+*/
+
+"use strict";
+
+const TOMBSTONE = Symbol("tombstone");
+const UNDEFINED_MARKER = Symbol("undefined");
+
+/**
+ * @template T
+ * @typedef {T | undefined} Cell<T>
+ */
+
+/**
+ * @template T
+ * @typedef {T | typeof TOMBSTONE | typeof UNDEFINED_MARKER} InternalCell<T>
+ */
+
+/**
+ * @template K
+ * @template V
+ * @param {[K, InternalCell<V>]} pair the internal cell
+ * @returns {[K, Cell<V>]} its “safe” representation
+ */
+const extractPair = pair => {
+	const key = pair[0];
+	const val = pair[1];
+	if (val === UNDEFINED_MARKER || val === TOMBSTONE) {
+		return [key, undefined];
+	} else {
+		return /** @type {[K, Cell<V>]} */ (pair);
+	}
+};
+
+/**
+ * @template K
+ * @template V
+ */
+class StackedMap {
+	/**
+	 * @param {Map<K, InternalCell<V>>[]=} parentStack an optional parent
+	 */
+	constructor(parentStack) {
+		/** @type {Map<K, InternalCell<V>>} */
+		this.map = new Map();
+		/** @type {Map<K, InternalCell<V>>[]} */
+		this.stack = parentStack === undefined ? [] : parentStack.slice();
+		this.stack.push(this.map);
+	}
+
+	/**
+	 * @param {K} item the key of the element to add
+	 * @param {V} value the value of the element to add
+	 * @returns {void}
+	 */
+	set(item, value) {
+		this.map.set(item, value === undefined ? UNDEFINED_MARKER : value);
+	}
+
+	/**
+	 * @param {K} item the item to delete
+	 * @returns {void}
+	 */
+	delete(item) {
+		if (this.stack.length > 1) {
+			this.map.set(item, TOMBSTONE);
+		} else {
+			this.map.delete(item);
+		}
+	}
+
+	/**
+	 * @param {K} item the item to test
+	 * @returns {boolean} true if the item exists in this set
+	 */
+	has(item) {
+		const topValue = this.map.get(item);
+		if (topValue !== undefined) {
+			return topValue !== TOMBSTONE;
+		}
+		if (this.stack.length > 1) {
+			for (let i = this.stack.length - 2; i >= 0; i--) {
+				const value = this.stack[i].get(item);
+				if (value !== undefined) {
+					this.map.set(item, value);
+					return value !== TOMBSTONE;
+				}
+			}
+			this.map.set(item, TOMBSTONE);
+		}
+		return false;
+	}
+
+	/**
+	 * @param {K} item the key of the element to return
+	 * @returns {Cell<V>} the value of the element
+	 */
+	get(item) {
+		const topValue = this.map.get(item);
+		if (topValue !== undefined) {
+			return topValue === TOMBSTONE || topValue === UNDEFINED_MARKER
+				? undefined
+				: topValue;
+		}
+		if (this.stack.length > 1) {
+			for (let i = this.stack.length - 2; i >= 0; i--) {
+				const value = this.stack[i].get(item);
+				if (value !== undefined) {
+					this.map.set(item, value);
+					return value === TOMBSTONE || value === UNDEFINED_MARKER
+						? undefined
+						: value;
+				}
+			}
+			this.map.set(item, TOMBSTONE);
+		}
+		return undefined;
+	}
+
+	_compress() {
+		if (this.stack.length === 1) return;
+		this.map = new Map();
+		for (const data of this.stack) {
+			for (const pair of data) {
+				if (pair[1] === TOMBSTONE) {
+					this.map.delete(pair[0]);
+				} else {
+					this.map.set(pair[0], pair[1]);
+				}
+			}
+		}
+		this.stack = [this.map];
+	}
+
+	asArray() {
+		this._compress();
+		return Array.from(this.map.keys());
+	}
+
+	asSet() {
+		this._compress();
+		return new Set(this.map.keys());
+	}
+
+	asPairArray() {
+		this._compress();
+		return Array.from(this.map.entries(), extractPair);
+	}
+
+	asMap() {
+		return new Map(this.asPairArray());
+	}
+
+	get size() {
+		this._compress();
+		return this.map.size;
+	}
+
+	createChild() {
+		return new StackedMap(this.stack);
+	}
+}
+
+module.exports = StackedMap;