Use generated .d.ts files

Author: schmidt-sebastian

packages/firestore/package.json

@@ -7,7 +7,7 @@
   "description": "The Cloud Firestore component of the Firebase JS SDK.",
   "author": "Firebase <[email protected]> (https://firebase.google.com/)",
   "scripts": {
-    "prebuild": "tsc -d --downlevelIteration --declarationDir dist/lib --emitDeclarationOnly && tsc -m es2015 --moduleResolution node scripts/*.ts ",
+    "prebuild": "tsc -m es2015 --moduleResolution node scripts/*.ts ",
     "build": "rollup -c rollup.config.es2017.js && rollup -c rollup.config.es5.js",
     "build:deps": "lerna run --scope @firebase/'{app,firestore}' --include-dependencies build",
     "build:console": "node tools/console.build.js",

packages/firestore/scripts/extract-api.ts

@@ -32,6 +32,17 @@ function extractIdentifiersFromNodeAndChildren(
   );
 }
 
+function extractTypeDeclaration(fileName: string): string {
+  const compilerOptions = {  declaration: true,
+    emitDeclarationOnly: true}
+  let dtsSource : string;
+  const host = ts.createCompilerHost( compilerOptions);
+  host.writeFile = (fileName: string, contents: string) => dtsSource = contents;
+  const program = ts.createProgram([fileName], compilerOptions, host);
+  program.emit()
+  return dtsSource!;
+}
+
 /**
  * Traverses TypeScript type definition files and returns the list of referenced
  * identifiers.
@@ -41,11 +52,16 @@ export function extractPublicIdentifiers(filePaths: string[]): Set<string> {
 
   for (const filePath of filePaths) {
     const contents = fs.readFileSync(filePath, { encoding: 'UTF-8' });
-    const sourceFile = ts.createSourceFile(
+    let sourceFile = ts.createSourceFile(
       filePath,
       contents,
       ts.ScriptTarget.ES2015
     );
+    
+    if (!sourceFile.isDeclarationFile) {
+     const dtsSource = extractTypeDeclaration(filePath);
+     sourceFile = ts.createSourceFile(filePath.replace(".ts", ".d.ts"), dtsSource, ts.ScriptTarget.ES2015);
+    }
 
     const identifiers = new Set<string>();
     ts.forEachChild(sourceFile, (childNode: ts.Node) =>