add externs to remove disabled warning

build.js

@@ -20,19 +20,19 @@ const closureCompiler = new ClosureCompiler({
   js: './src/*.js',
   entry_point: './src/index.js',
   js_output_file: './proxy.min.js',
-  language_in: 'ECMASCRIPT6',
+  language_in: 'ECMASCRIPT6_STRICT',
   language_out: 'ECMASCRIPT5',
   compilation_level: 'ADVANCED_OPTIMIZATIONS',
   dependency_mode: 'STRICT',
+  warning_level: 'VERBOSE',
   process_common_js_modules: true,
   output_wrapper: '(function(){%output%})()', // this prevents closure compiler from polluting the global scope
-  jscomp_off: 'undefinedVars', // `global` and `process` are not defined...
 });
 
 const compilerProcess = closureCompiler.run((code, stdout, stderr) => {
   if (stderr) {
-    console.error('err!', stdErr);
+    console.error('err!', stderr);
     return;
   }
-  console.log('done!');
+  console.log('done!', stdout);
 });

src/externs.js

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2018 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+/**
+ * @externs
+ * @fileoverview Externs so that Closure doesn't complain about environment checks.
+ */
+
+/**
+ * @type {Object}
+ */
+var process;
+
+/**
+ * @type {Object}
+ */
+var global;