Make it as api so we can test it

src/compiler/program.ts

@@ -74,12 +74,7 @@ namespace ts {
     // TODO(shkamat): update this after reworking ts build API
     export function createCompilerHostWorker(options: CompilerOptions, setParentNodes?: boolean, system = sys): CompilerHost {
         const existingDirectories = createMap<boolean>();
-        function getCanonicalFileName(fileName: string): string {
-            // if underlying system can distinguish between two files whose names differs only in cases then file name already in canonical form.
-            // otherwise use toLowerCase as a canonical form.
-            return system.useCaseSensitiveFileNames ? fileName : fileName.toLowerCase();
-        }
-
+        const getCanonicalFileName = createGetCanonicalFileName(system.useCaseSensitiveFileNames);
         function getSourceFile(fileName: string, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile | undefined {
             let text: string | undefined;
             try {

src/compiler/watch.ts

@@ -383,6 +383,56 @@ namespace ts {
         if (!buildInfo.program) return undefined;
         return createBuildProgramUsingProgramBuildInfo(buildInfo.program);
     }
+
+    export function createIncrementalCompilerHost(options: CompilerOptions, system = sys): CompilerHost {
+        const host = createCompilerHostWorker(options, /*setParentNodes*/ undefined, system);
+        host.createHash = maybeBind(system, system.createHash);
+        setGetSourceFileAsHashVersioned(host, system);
+        changeCompilerHostLikeToUseCache(host, fileName => toPath(fileName, host.getCurrentDirectory(), host.getCanonicalFileName));
+        return host;
+    }
+
+    export interface IncrementalProgramOptions<T extends BuilderProgram> {
+        rootNames: ReadonlyArray<string>;
+        options: CompilerOptions;
+        configFileParsingDiagnostics?: ReadonlyArray<Diagnostic>;
+        projectReferences?: ReadonlyArray<ProjectReference>;
+        host?: CompilerHost;
+        createProgram?: CreateProgram<T>;
+    }
+    export function createIncrementalProgram<T extends BuilderProgram = EmitAndSemanticDiagnosticsBuilderProgram>({
+        rootNames, options, configFileParsingDiagnostics, projectReferences, host, createProgram
+    }: IncrementalProgramOptions<T>): T {
+        host = host || createIncrementalCompilerHost(options);
+        createProgram = createProgram || createEmitAndSemanticDiagnosticsBuilderProgram as any as CreateProgram<T>;
+        const oldProgram = readBuilderProgram(options, path => host!.readFile(path)) as any as T;
+        return createProgram(rootNames, options, host, oldProgram, configFileParsingDiagnostics, projectReferences);
+    }
+
+    export interface IncrementalCompilationOptions {
+        rootNames: ReadonlyArray<string>;
+        options: CompilerOptions;
+        configFileParsingDiagnostics?: ReadonlyArray<Diagnostic>;
+        projectReferences?: ReadonlyArray<ProjectReference>;
+        host?: CompilerHost;
+        reportDiagnostic?: DiagnosticReporter;
+        reportErrorSummary?: ReportEmitErrorSummary;
+        afterProgramEmitAndDiagnostics?(program: EmitAndSemanticDiagnosticsBuilderProgram): void;
+        system?: System;
+    }
+    export function performIncrementalCompilation(input: IncrementalCompilationOptions) {
+        const system = input.system || sys;
+        const host = input.host || (input.host = createIncrementalCompilerHost(input.options, system));
+        const builderProgram = createIncrementalProgram(input);
+        const exitStatus = emitFilesAndReportErrors(
+            builderProgram,
+            input.reportDiagnostic || createDiagnosticReporter(system),
+            s => host!.trace && host!.trace!(s),
+            input.reportErrorSummary || input.options.pretty ? errorCount => system.write(getErrorSummaryText(errorCount, system.newLine)) : undefined
+        );
+        if (input.afterProgramEmitAndDiagnostics) input.afterProgramEmitAndDiagnostics(builderProgram);
+        return exitStatus;        
+    }
 }
 
 namespace ts {