Handle @internal APIs.

With stripInternal set, TypeScript does not generate exports for APIs
marked as @internal, which tsickle should respect when generating Clutz
aliases.

src/tsickle.ts

@@ -171,7 +171,7 @@ export function emit(
                 sourceFiles.map(sf => sf.fileName)}`);
           }
           const originalSource = sourceFiles[0];
-          content = addClutzAliases(content, originalSource, typeChecker, host);
+          content = addClutzAliases(content, originalSource, typeChecker, host, tsOptions);
         }
         writeFile(fileName, content, writeByteOrderMark, onError, sourceFiles);
       };
@@ -229,7 +229,7 @@ function stringCompare(a: string, b: string): number {
  */
 function addClutzAliases(
     dtsFileContent: string, sourceFile: ts.SourceFile, typeChecker: ts.TypeChecker,
-    host: TsickleHost): string {
+    host: TsickleHost, options: ts.CompilerOptions): string {
   const moduleSymbol = typeChecker.getSymbolAtLocation(sourceFile);
   const moduleExports = moduleSymbol && typeChecker.getExportsOfModule(moduleSymbol);
   if (!moduleExports) return dtsFileContent;
@@ -278,6 +278,14 @@ function addClutzAliases(
         return false;
       }
 
+      // @internal marked APIs are not exported, so must not get aliases.
+      // This uses an internal TS API, assuming that accessing this will be more stable compared to
+      // implementing our own version.
+      // tslint:disable-next-line: no-any
+      if (options.stripInternal && (ts as any).isInternalDeclaration(d, origSourceFile)) {
+        return false;
+      }
+
       if (!ts.isExportSpecifier(d)) {
         // we have a pure export (case 1) thus safe to emit clutz alias.
         return true;

test/test_support.ts

@@ -56,6 +56,7 @@ export const baseCompilerOptions: ts.CompilerOptions = {
   allowJs: false,
   importHelpers: true,
   noEmitHelpers: true,
+  stripInternal: true,
   baseUrl: '.',
   paths: {
     // The compiler builtin 'tslib' library is looked up by name,

test_files/internal.declaration/internal.d.ts

@@ -0,0 +1,5 @@
+/**
+ * @fileoverview Test to reproduce that \@internal functions are not re-exported for Clutz. There
+ * should not be any `.d.ts` aliases generated for the function below.
+ */
+export {};

test_files/internal.declaration/internal.ts

@@ -0,0 +1,9 @@
+/**
+ * @fileoverview Test to reproduce that \@internal functions are not re-exported for Clutz. There
+ * should not be any `.d.ts` aliases generated for the function below.
+ */
+
+/** @internal */
+export function internalFunction() {
+  return 42;
+}