Undo potentially compatibility-destroying changes

src/compiler/sys.ts

@@ -131,6 +131,9 @@ let pollingChunkSize = createPollingIntervalBasedLevels(defaultChunkLevels);
 export let unchangedPollThresholds = createPollingIntervalBasedLevels(defaultChunkLevels);
 
 function setCustomPollingValues(system: System) {
+    if (system.getEnvironmentVariable === undefined) {
+        return; 
+    }
     const pollingIntervalChanged = setCustomLevels("TSC_WATCH_POLLINGINTERVAL", PollingInterval);
     pollingChunkSize = getCustomPollingBasedLevels("TSC_WATCH_POLLINGCHUNKSIZE", defaultChunkLevels) || pollingChunkSize;
     unchangedPollThresholds = getCustomPollingBasedLevels("TSC_WATCH_UNCHANGEDPOLLTHRESHOLDS", defaultChunkLevels) || unchangedPollThresholds;

src/services/stringCompletions.ts

@@ -1182,6 +1182,7 @@ function getCompletionEntriesFromTypings(host: LanguageServiceHost, options: Com
 }
 
 function enumerateNodeModulesVisibleToScript(host: LanguageServiceHost, scriptPath: string): readonly string[] {
+    if (host.readFile === undefined || host.fileExists === undefined) return emptyArray;
     const result: string[] = [];
     for (const packageJson of findPackageJsons(scriptPath, host)) {
         const contents = readJson(packageJson, host as { readFile: (filename: string) => string | undefined; }); // Cast to assert that readFile is defined

src/services/utilities.ts

@@ -3670,6 +3670,9 @@ export function findPackageJson(directory: string, host: LanguageServiceHost): s
 
 /** @internal */
 export function getPackageJsonsVisibleToFile(fileName: string, host: LanguageServiceHost): readonly ProjectPackageJsonInfo[] {
+    if (host.fileExists === undefined) {
+        return [];
+    }
     const packageJsons: ProjectPackageJsonInfo[] = [];
     forEachAncestorDirectory(getDirectoryPath(fileName), ancestor => {
         const packageJsonFileName = combinePaths(ancestor, "package.json");