Implement fix for Fast Refresh bug

lib/DirectoryWatcher.js

@@ -738,7 +738,27 @@ class DirectoryWatcher extends EventEmitter {
 				for (const watcher of watchers) {
 					const path = watcher.path;
 					if (!fileTimestamps.has(path)) {
-						fileTimestamps.set(path, null);
+						// If the file does not exist, we'll mark it as null,
+						// but beforehand we check if the same file with different
+						// casing exists in the map.
+
+						// This can happen because the `this.files` map is case-sensitive.
+						let lowercase = withoutCase(path);
+						if (this.filesWithoutCase.has(lowercase)) {
+							const keys = fileTimestamps.keys();
+							let nextKey = null;
+							while (!(nextKey = keys.next()).done) {
+								if (nextKey.value.toLowerCase() === lowercase) {
+									fileTimestamps.set(path, fileTimestamps.get(nextKey.value));
+									break;
+								}
+							}
+						}
+
+						// If no file with different casing was found, we'll mark it as null
+						if (!fileTimestamps.has(path)) {
+							fileTimestamps.set(path, null);
+						}
 					}
 				}
 			}

test/DirectoryWatcher.js

@@ -12,6 +12,15 @@ var testHelper = new TestHelper(fixtures);
 
 var openWatchers = [];
 
+var fsIsCaseInsensitive;
+try {
+	fsIsCaseInsensitive = require("fs").existsSync(
+		path.join(__dirname, "..", "PACKAGE.JSON")
+	);
+} catch (e) {
+	fsIsCaseInsensitive = false;
+}
+
 var DirectoryWatcher = function(p, options) {
 	var d = new OrgDirectoryWatcher(getWatcherManager(options), p, options);
 	openWatchers.push(d);
@@ -169,6 +178,34 @@ describe("DirectoryWatcher", function() {
 		});
 	});
 
+	if (fsIsCaseInsensitive) {
+		it("should collectTimeInfoEntries for uppercase and lowercase filename", function(done) {
+			testHelper.file("A");
+			var d = new DirectoryWatcher(fixtures, {});
+			var a = d.watch(path.join(fixtures, "a"));
+
+			let filenameTimestamps = new Map();
+			let directoryTimestamps = new Map();
+			testHelper.tick(function() {
+				d.collectTimeInfoEntries(filenameTimestamps, directoryTimestamps);
+				let lowercase = filenameTimestamps.get(
+					path.join(testHelper.testdir, "a")
+				);
+				let uppercase = filenameTimestamps.get(
+					path.join(testHelper.testdir, "A")
+				);
+				if (!lowercase) {
+					throw new Error("should have timeInfoEntries for lowercase");
+				}
+				if (!uppercase) {
+					throw new Error("should have timeInfoEntries for uppercase");
+				}
+				a.close();
+				done();
+			});
+		});
+	}
+
 	if (!+process.env.WATCHPACK_POLLING) {
 		it("should log errors emitted from watcher to stderr", function(done) {
 			var error_logged = false;