ci(dart): Uncomment dart.dev build and make it required

Workaround for https://github.com/dart-lang/dartdoc/issues/1099: remove
the `--input=.` parameter to `dartdoc`.

Closes #6823, #6410

Closes #6958

tools/build/dartapidocs.js

@@ -21,7 +21,7 @@ module.exports = function(gulp, plugins, config) {
     } else {
       console.log('INFO: running dartdoc for ', dir);
 
-      var stream = spawn(config.command, ['--input=.', '--output=' + config.output],
+      var stream = spawn(config.command, ['--output=' + config.output],
                          {stdio: [process.stdin, process.stdout, process.stderr], cwd: dir});
 
       stream.on('exit', function(code) {