feat: warn if BABEL_POLYFILL is set to none (umijs#1844)

* feat: warn user if BABEL_POLYFILL is set to none

* chore: don't test 11

.travis.yml

@@ -3,7 +3,6 @@ language: node_js
 node_js:
   - "8"
   - "10"
-  - "11"
 
 before_script:
   - npm run build

docs/guide/env-variables.md

@@ -73,8 +73,6 @@ ANALYZE server port, default 8888.
 
 ### BABEL_POLYFILL <Badge text="2.2.0+"/>
 
-默认引入 `@babel/polyfill`，值为 none 时不引入。比如：
-
 `@babel/polyfill` is included by default, set the value to `none` if you don't want it.
 
 e.g.

docs/plugin/develop.md

@@ -257,7 +257,7 @@ api.log.pending('Write release notes for %s', '1.2.0');
 api.log.watch('Recursively watching build directory...');
 ```
 
-Output various types of logs.
+Output various [types](https://github.com/klaussinani/signale/blob/94984998a0e9cb280e68959ddd9db70b49713738/types.js#L4) of logs.
 
 ### winPath
 

docs/zh/plugin/develop.md

@@ -257,7 +257,7 @@ api.log.pending('Write release notes for %s', '1.2.0');
 api.log.watch('Recursively watching build directory...');
 ```
 
-输出各类日志。
+输出[各种类型](https://github.com/klaussinani/signale/blob/94984998a0e9cb280e68959ddd9db70b49713738/types.js#L4)的日志。
 
 ### winPath
 

packages/umi-build-dev/src/plugins/targets.js

@@ -1,10 +1,11 @@
 import { join, relative } from 'path';
 import { readFileSync } from 'fs';
+import chalk from 'chalk';
 import { isPlainObject } from 'lodash';
 import assert from 'assert';
 
 export default function(api) {
-  const { paths, winPath } = api;
+  const { paths, winPath, log } = api;
 
   api._registerConfig(() => {
     return () => {
@@ -53,6 +54,13 @@ export default function(api) {
         },
       ];
     } else {
+      log.warn(
+        chalk.yellow(
+          `Since you have configured the environment variable ${chalk.bold(
+            'BABEL_POLYFILL',
+          )} to none, no patches will be included.`,
+        ),
+      );
       return [];
     }
   });