Resolved warning in tsconfig.

Author: Kiyotoshi Ichikawa

tsconfig.json

@@ -4,7 +4,7 @@
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
     "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     "skipLibCheck": true,
-    "lib": ["ES2015"],                             /* Specify library files to be included in the compilation:  */
+    "lib": ["es2015"],                             /* Specify library files to be included in the compilation:  */
      "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
     // "jsx": "preserve",                     /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */