Use gulp and babel to make es5 output

.nvmrc

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+10.12.0

gulpfile.js

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+const gulp = require('gulp');
+const babel = require('gulp-babel');
+const sourcemaps = require('gulp-sourcemaps');
+const ts = require('gulp-typescript');
+const gulpif = require('gulp-if');
+
+const tsProj = ts.createProject('tsconfig.json');
+
+const babelConfig = {
+  "presets": [
+    // Target supported browser environments
+    ["@babel/preset-env", { "targets": "> 0.25%, not dead" }]
+  ],
+  "plugins": [
+    // Enable generator runtime support
+    ["@babel/plugin-transform-runtime", { "regenerator": true }]
+  ],
+};
+
+function build() {
+  return gulp.src('src/**/*.ts')
+    .pipe(sourcemaps.init())
+    .pipe(tsProj())
+    .pipe(gulpif(/[.]js$/, babel(babelConfig)))
+    .pipe(sourcemaps.write('.'))
+    .pipe(gulp.dest('target/src'));
+};
+
+exports.default = build;