diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index abae979a25d..e18ce2efeeb 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -5336,7 +5336,7 @@ namespace ts {
             while (i > 0) {
                 i--;
                 if (isSubtypeOfAny(types[i], types)) {
-                    removeItemAt(types, i);
+                    removeItemAtPreservingOrder(types, i);
                 }
             }
         }
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index dd4ba822243..73c92eff547 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1395,7 +1395,7 @@ namespace ts {
     }
 
     /** Remove an item from an array, moving everything to its right one space left. */
-    export function removeItemAt<T>(array: T[], index: number): void {
+    export function removeItemAtPreservingOrder<T>(array: T[], index: number): void {
         // This seems to be faster than either `array.splice(i, 1)` or `array.copyWithin(i, i+ 1)`.
         for (let i = index; i < array.length - 1; i++) {
             array[i] = array[i + 1];
@@ -1403,23 +1403,29 @@ namespace ts {
         array.pop();
     }
 
+    export function unorderedRemoveItemAt<T>(array: T[], index: number): void {
+        // Fill in the "hole" left at `index`.
+        array[index] = array[array.length - 1];
+        array.pop();
+    }
+
     /** Remove the *first* occurrence of `item` from the array. */
-    export function removeItem<T>(item: T, array: T[]): void {
-        removeFirstItemWhere(array, element => element === item);
+    export function unorderedRemoveItem<T>(item: T, array: T[]): void {
+        unorderedRemoveFirstItemWhere(array, element => element === item);
     }
 
     /** Remove the *first* element satisfying `predicate`. */
-    export function removeFirstItemWhere<T>(array: T[], predicate: (element: T) => boolean): void {
+    export function unorderedRemoveFirstItemWhere<T>(array: T[], predicate: (element: T) => boolean): void {
         for (let i = 0; i < array.length; i++) {
             if (predicate(array[i])) {
-                removeItemAt(array, i);
+                unorderedRemoveItemAt(array, i);
                 break;
             }
         }
     }
 
-    export function createGetCanonicalFileName(useCaseSensitivefileNames: boolean): (fileName: string) => string {
-        return useCaseSensitivefileNames
+    export function createGetCanonicalFileName(useCaseSensitiveFileNames: boolean): (fileName: string) => string {
+        return useCaseSensitiveFileNames
             ? ((fileName) => fileName)
             : ((fileName) => fileName.toLowerCase());
     }
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 7953a2f42e8..966a282ae62 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -285,7 +285,7 @@ namespace ts {
                 function removeFileWatcherCallback(filePath: string, callback: FileWatcherCallback) {
                     const callbacks = fileWatcherCallbacks[filePath];
                     if (callbacks) {
-                        removeItem(callback, callbacks);
+                        unorderedRemoveItem(callback, callbacks);
                         if (callbacks.length === 0) {
                             delete fileWatcherCallbacks[filePath];
                         }
diff --git a/src/compiler/tsc.ts b/src/compiler/tsc.ts
index 02a97c01132..bd3f88676bc 100644
--- a/src/compiler/tsc.ts
+++ b/src/compiler/tsc.ts
@@ -490,7 +490,7 @@ namespace ts {
             sourceFile.fileWatcher.close();
             sourceFile.fileWatcher = undefined;
             if (removed) {
-                removeItem(sourceFile.fileName, rootFileNames);
+                unorderedRemoveItem(sourceFile.fileName, rootFileNames);
             }
             startTimerForRecompilation();
         }
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 3b36381146f..634c159dbb8 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -1558,7 +1558,7 @@ namespace Harness {
                     tsConfig.options.configFilePath = data.name;
 
                     // delete entry from the list
-                    ts.removeItemAt(testUnitData, i);
+                    ts.removeItemAtPreservingOrder(testUnitData, i);
 
                     break;
                 }
diff --git a/src/harness/unittests/tsserverProjectSystem.ts b/src/harness/unittests/tsserverProjectSystem.ts
index cd87164245a..b02e4d81cd3 100644
--- a/src/harness/unittests/tsserverProjectSystem.ts
+++ b/src/harness/unittests/tsserverProjectSystem.ts
@@ -214,7 +214,7 @@ namespace ts {
                 referenceCount: 0,
                 directoryName,
                 close: () => {
-                    removeFirstItemWhere(callbacks, cb => cb.cb === callback);
+                    unorderedRemoveFirstItemWhere(callbacks, cb => cb.cb === callback);
                     if (!callbacks.length) {
                         delete this.watchedDirectories[path];
                     }
@@ -248,7 +248,7 @@ namespace ts {
             callbacks.push(callback);
             return {
                 close: () => {
-                    removeItem(callback, callbacks);
+                    unorderedRemoveItem(callback, callbacks);
                     if (!callbacks.length) {
                         delete this.watchedFiles[path];
                     }
@@ -263,7 +263,7 @@ namespace ts {
         };
         readonly clearTimeout = (timeoutId: any): void => {
             if (typeof timeoutId === "number") {
-                removeItemAt(this.callbackQueue, timeoutId);
+                unorderedRemoveItemAt(this.callbackQueue, timeoutId);
             }
         };
 
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 0a12bf68d79..0aa10819c49 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -275,7 +275,7 @@ namespace ts.server {
         removeRoot(info: ScriptInfo) {
             if (this.filenameToScript.contains(info.path)) {
                 this.filenameToScript.remove(info.path);
-                removeItem(info, this.roots);
+                unorderedRemoveItem(info, this.roots);
                 this.resolvedModuleNames.remove(info.path);
                 this.resolvedTypeReferenceDirectives.remove(info.path);
             }
@@ -849,7 +849,7 @@ namespace ts.server {
                 project.directoryWatcher.close();
                 forEachValue(project.directoriesWatchedForWildcards, watcher => { watcher.close(); });
                 delete project.directoriesWatchedForWildcards;
-                removeItem(project, this.configuredProjects);
+                unorderedRemoveItem(project, this.configuredProjects);
             }
             else {
                 for (const directory of project.directoriesWatchedForTsconfig) {
@@ -861,7 +861,7 @@ namespace ts.server {
                         delete project.projectService.directoryWatchersForTsconfig[directory];
                     }
                 }
-                removeItem(project, this.inferredProjects);
+                unorderedRemoveItem(project, this.inferredProjects);
             }
 
             const fileNames = project.getFileNames();
@@ -986,7 +986,7 @@ namespace ts.server {
                 }
             }
             else {
-                removeItem(info, this.openFilesReferenced);
+                unorderedRemoveItem(info, this.openFilesReferenced);
             }
             info.close();
         }
@@ -1496,13 +1496,13 @@ namespace ts.server {
                             // openFileRoots or openFileReferenced.
                             if (info.isOpen) {
                                 if (this.openFileRoots.indexOf(info) >= 0) {
-                                    removeItem(info, this.openFileRoots);
+                                    unorderedRemoveItem(info, this.openFileRoots);
                                     if (info.defaultProject && !info.defaultProject.isConfiguredProject()) {
                                         this.removeProject(info.defaultProject);
                                     }
                                 }
                                 if (this.openFilesReferenced.indexOf(info) >= 0) {
-                                    removeItem(info, this.openFilesReferenced);
+                                    unorderedRemoveItem(info, this.openFilesReferenced);
                                 }
                                 this.openFileRootsConfigured.push(info);
                                 info.defaultProject = project;