Add script to generate TypeScript declarations.

Includes hand written types for cases that couldn't be generated.

Uses https://github.com/Polymer/gen-typescript-declarations.

Run with `gulp generate-typescript`.

Also asyncify the gulpfile with fs-extra.

gen-tsd.json

@@ -0,0 +1,18 @@
+{
+  "exclude": [
+    "dist/",
+    "externs/",
+    "gulpfile.js",
+    "test/",
+    "util/"
+  ],
+  "removeReferences": [
+    "../shadycss/apply-shim.d.ts",
+    "../shadycss/custom-style-interface.d.ts"
+  ],
+  "addReferences": {
+    "lib/utils/boot.d.ts": [
+      "extra-types.d.ts"
+    ]
+  }
+}

gulpfile.js

@@ -16,7 +16,7 @@ const gulpif = require('gulp-if');
 const runseq = require('run-sequence');
 const del = require('del');
 const eslint = require('gulp-eslint');
-const fs = require('fs');
+const fs = require('fs-extra');
 const path = require('path');
 const mergeStream = require('merge-stream');
 const babel = require('gulp-babel');
@@ -260,15 +260,24 @@ gulp.task('lint', (done) => {
   runseq('lint-eslint', 'lint-closure', done);
 });
 
-gulp.task('generate-externs', ['clean'], () => {
+gulp.task('generate-externs', ['clean'], async () => {
   let genClosure = require('@polymer/gen-closure-declarations').generateDeclarations;
-  return genClosure().then((declarations) => {
-    fs.writeFileSync('externs/closure-types.js', `${header}${declarations}`);
-  });
+  const declarations = await genClosure();
+  await fs.writeFile('externs/closure-types.js', `${header}${declarations}`);
+});
+
+gulp.task('generate-typescript', async () => {
+  let genTs = require('@polymer/gen-typescript-declarations').generateDeclarations;
+  await del(['types/**/*.d.ts', '!types/extra-types.d.ts']);
+  const config = await fs.readJson('gen-tsd.json');
+  const files = await genTs('.', config);
+  for (const [filePath, contents] of files) {
+    await fs.outputFile(path.join('types', filePath), contents);
+  }
 });
 
 gulp.task('update-version', () => {
   gulp.src('lib/utils/boot.html')
   .pipe(replace(/(window.Polymer.version = )'\d+\.\d+\.\d+'/, `$1'${require('./package.json').version}'`))
   .pipe(gulp.dest('lib/utils'));
-});
+});