diff --git a/src/compiler/resolutionCache.ts b/src/compiler/resolutionCache.ts
index b22d90a76f646..6c8c79adefb81 100644
--- a/src/compiler/resolutionCache.ts
+++ b/src/compiler/resolutionCache.ts
@@ -79,7 +79,7 @@ namespace ts {
     export function createResolutionCache(resolutionHost: ResolutionCacheHost, rootDirForResolution: string | undefined, logChangesWhenResolvingModule: boolean): ResolutionCache {
         let filesWithChangedSetOfUnresolvedImports: Path[] | undefined;
         let filesWithInvalidatedResolutions: Map<true> | undefined;
-        let filesWithInvalidatedNonRelativeUnresolvedImports: Map<ReadonlyArray<string>> | undefined;
+        let filesWithInvalidatedNonRelativeUnresolvedImports: ReadonlyMap<ReadonlyArray<string>> | undefined;
         let allFilesHaveInvalidatedResolution = false;
         const nonRelativeExternalModuleResolutions = createMultiMap<ResolutionWithFailedLookupLocations>();
 
@@ -241,14 +241,14 @@ namespace ts {
         }
 
         function resolveNamesWithLocalCache<T extends ResolutionWithFailedLookupLocations, R extends ResolutionWithResolvedFileName>(
-            names: string[],
+            names: ReadonlyArray<string>,
             containingFile: string,
             redirectedReference: ResolvedProjectReference | undefined,
             cache: Map<Map<T>>,
             perDirectoryCacheWithRedirects: CacheWithRedirects<Map<T>>,
             loader: (name: string, containingFile: string, options: CompilerOptions, host: ModuleResolutionHost, redirectedReference?: ResolvedProjectReference) => T,
             getResolutionWithResolvedFileName: GetResolutionWithResolvedFileName<T, R>,
-            reusedNames: string[] | undefined,
+            reusedNames: ReadonlyArray<string> | undefined,
             logChanges: boolean): (R | undefined)[] {
 
             const path = resolutionHost.toPath(containingFile);
@@ -675,7 +675,7 @@ namespace ts {
             );
         }
 
-        function setFilesWithInvalidatedNonRelativeUnresolvedImports(filesMap: Map<ReadonlyArray<string>>) {
+        function setFilesWithInvalidatedNonRelativeUnresolvedImports(filesMap: ReadonlyMap<ReadonlyArray<string>>) {
             Debug.assert(filesWithInvalidatedNonRelativeUnresolvedImports === filesMap || filesWithInvalidatedNonRelativeUnresolvedImports === undefined);
             filesWithInvalidatedNonRelativeUnresolvedImports = filesMap;
         }
diff --git a/src/harness/virtualFileSystemWithWatch.ts b/src/harness/virtualFileSystemWithWatch.ts
index d2b7c7869c568..bd31a04a0df13 100644
--- a/src/harness/virtualFileSystemWithWatch.ts
+++ b/src/harness/virtualFileSystemWithWatch.ts
@@ -128,7 +128,7 @@ interface Array<T> {}`
         return s && isString((<FsSymLink>s).symLink);
     }
 
-    function invokeWatcherCallbacks<T>(callbacks: T[], invokeCallback: (cb: T) => void): void {
+    function invokeWatcherCallbacks<T>(callbacks: ReadonlyArray<T> | undefined, invokeCallback: (cb: T) => void): void {
         if (callbacks) {
             // The array copy is made to ensure that even if one of the callback removes the callbacks,
             // we dont miss any callbacks following it
@@ -650,15 +650,15 @@ interface Array<T> {}`
 
         // For overriding the methods
         invokeWatchedDirectoriesCallback(folderFullPath: string, relativePath: string) {
-            invokeWatcherCallbacks(this.watchedDirectories.get(this.toPath(folderFullPath))!, cb => this.directoryCallback(cb, relativePath));
+            invokeWatcherCallbacks(this.watchedDirectories.get(this.toPath(folderFullPath)), cb => this.directoryCallback(cb, relativePath));
         }
 
         invokeWatchedDirectoriesRecursiveCallback(folderFullPath: string, relativePath: string) {
-            invokeWatcherCallbacks(this.watchedDirectoriesRecursive.get(this.toPath(folderFullPath))!, cb => this.directoryCallback(cb, relativePath));
+            invokeWatcherCallbacks(this.watchedDirectoriesRecursive.get(this.toPath(folderFullPath)), cb => this.directoryCallback(cb, relativePath));
         }
 
         private invokeFileWatcher(fileFullPath: string, eventKind: FileWatcherEventKind, useFileNameInCallback?: boolean) {
-            invokeWatcherCallbacks(this.watchedFiles.get(this.toPath(fileFullPath))!, ({ cb, fileName }) => cb(useFileNameInCallback ? fileName : fileFullPath, eventKind));
+            invokeWatcherCallbacks(this.watchedFiles.get(this.toPath(fileFullPath)), ({ cb, fileName }) => cb(useFileNameInCallback ? fileName : fileFullPath, eventKind));
         }
 
         private getRelativePathToDirectory(directoryFullPath: string, fileFullPath: string) {
diff --git a/src/server/project.ts b/src/server/project.ts
index ff2e3539b6bbc..35ccea6600818 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -795,41 +795,6 @@ namespace ts.server {
             }
         }
 
-        /* @internal */
-        private extractUnresolvedImportsFromSourceFile(file: SourceFile, ambientModules: string[]): ReadonlyArray<string> {
-            const cached = this.cachedUnresolvedImportsPerFile.get(file.path);
-            if (cached) {
-                // found cached result, return
-                return cached;
-            }
-            let unresolvedImports: string[] | undefined;
-            if (file.resolvedModules) {
-                file.resolvedModules.forEach((resolvedModule, name) => {
-                    // pick unresolved non-relative names
-                    if (!resolvedModule && !isExternalModuleNameRelative(name) && !isAmbientlyDeclaredModule(name)) {
-                        // for non-scoped names extract part up-to the first slash
-                        // for scoped names - extract up to the second slash
-                        let trimmed = name.trim();
-                        let i = trimmed.indexOf("/");
-                        if (i !== -1 && trimmed.charCodeAt(0) === CharacterCodes.at) {
-                            i = trimmed.indexOf("/", i + 1);
-                        }
-                        if (i !== -1) {
-                            trimmed = trimmed.substr(0, i);
-                        }
-                        (unresolvedImports || (unresolvedImports = [])).push(trimmed);
-                    }
-                });
-            }
-
-            this.cachedUnresolvedImportsPerFile.set(file.path, unresolvedImports || emptyArray);
-            return unresolvedImports || emptyArray;
-
-            function isAmbientlyDeclaredModule(name: string) {
-                return ambientModules.some(m => m === name);
-            }
-        }
-
         /* @internal */
         onFileAddedOrRemoved() {
             this.hasAddedorRemovedFiles = true;
@@ -863,15 +828,7 @@ namespace ts.server {
                 // (can reuse cached imports for files that were not changed)
                 // 4. compilation settings were changed in the way that might affect module resolution - drop all caches and collect all data from the scratch
                 if (hasNewProgram || changedFiles.length) {
-                    let result: string[] | undefined;
-                    const ambientModules = this.program.getTypeChecker().getAmbientModules().map(mod => stripQuotes(mod.getName()));
-                    for (const sourceFile of this.program.getSourceFiles()) {
-                        const unResolved = this.extractUnresolvedImportsFromSourceFile(sourceFile, ambientModules);
-                        if (unResolved !== emptyArray) {
-                            (result || (result = [])).push(...unResolved);
-                        }
-                    }
-                    this.lastCachedUnresolvedImportsList = result ? sortAndDeduplicate(result) : emptyArray;
+                    this.lastCachedUnresolvedImportsList = getUnresolvedImports(this.program, this.cachedUnresolvedImportsPerFile);
                 }
 
                 this.projectService.typingsCache.enqueueInstallTypingsForProject(this, this.lastCachedUnresolvedImportsList, hasAddedorRemovedFiles);
@@ -1218,6 +1175,25 @@ namespace ts.server {
         }
     }
 
+    function getUnresolvedImports(program: Program, cachedUnresolvedImportsPerFile: Map<ReadonlyArray<string>>): SortedReadonlyArray<string> {
+        const ambientModules = program.getTypeChecker().getAmbientModules().map(mod => stripQuotes(mod.getName()));
+        return sortAndDeduplicate(flatMap(program.getSourceFiles(), sourceFile =>
+            extractUnresolvedImportsFromSourceFile(sourceFile, ambientModules, cachedUnresolvedImportsPerFile)));
+    }
+    function extractUnresolvedImportsFromSourceFile(file: SourceFile, ambientModules: ReadonlyArray<string>, cachedUnresolvedImportsPerFile: Map<ReadonlyArray<string>>): ReadonlyArray<string> {
+        return getOrUpdate(cachedUnresolvedImportsPerFile, file.path, () => {
+            if (!file.resolvedModules) return emptyArray;
+            let unresolvedImports: string[] | undefined;
+            file.resolvedModules.forEach((resolvedModule, name) => {
+                // pick unresolved non-relative names
+                if (!resolvedModule && !isExternalModuleNameRelative(name) && !ambientModules.some(m => m === name)) {
+                    unresolvedImports = append(unresolvedImports, parsePackageName(name).packageName);
+                }
+            });
+            return unresolvedImports || emptyArray;
+        });
+    }
+
     /**
      * If a file is opened and no tsconfig (or jsconfig) is found,
      * the file and its imports/references are put into an InferredProject.