chore(tools/gulp): add html minifying task to generated docs (#3476)

tools/gulp/tasks/docs.ts

@@ -1,6 +1,7 @@
 import {task, src, dest} from 'gulp';
 import {Dgeni} from 'dgeni';
 import * as path from 'path';
+import {HTML_MINIFIER_OPTIONS} from '../constants';
 
 // Node packages that lack of types.
 const markdown = require('gulp-markdown');
@@ -8,6 +9,7 @@ const transform = require('gulp-transform');
 const highlight = require('gulp-highlight-files');
 const rename = require('gulp-rename');
 const flatten = require('gulp-flatten');
+const htmlmin = require('gulp-htmlmin');
 const hljs = require('highlight.js');
 const dom  = require('gulp-dom');
 
@@ -42,7 +44,7 @@ const MARKDOWN_TAGS_TO_CLASS_ALIAS = [
   'ul'
 ];
 
-task('docs', ['markdown-docs', 'highlight-docs', 'api-docs']);
+task('docs', ['markdown-docs', 'highlight-docs', 'api-docs', 'minify-html-docs']);
 
 task('markdown-docs', () => {
   return src(['src/lib/**/*.md', 'guides/*.md'])
@@ -83,6 +85,12 @@ task('api-docs', () => {
   return docs.generate();
 });
 
+task('minify-html-docs', ['api-docs'], () => {
+  return src('dist/docs/api/*.html')
+    .pipe(htmlmin(HTML_MINIFIER_OPTIONS))
+    .pipe(dest('dist/docs/api/'));
+});
+
 /** Updates the markdown file's content to work inside of the docs app. */
 function transformMarkdownFiles(buffer: Buffer, file: any): string {
   let content = buffer.toString('utf-8');