Add a trivial polyfill for Number.parseInt.

It's in the ES6 externs, so JSCompiler doesn't complain when it sees calls to this function.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170741903

src/com/google/javascript/jscomp/js/es6/number/parseint.js

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2017 The Closure Compiler Authors.
+ *
+ * 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.
+ */
+
+'require util/polyfill';
+
+$jscomp.polyfill('Number.parseInt', function(orig) {
+  return orig || parseInt;
+}, 'es6', 'es3');

src/com/google/javascript/jscomp/js/polyfills.txt

@@ -32,6 +32,7 @@ Number.isFinite es6 es3 es6/number/isfinite
 Number.isInteger es6 es3 es6/number/isinteger
 Number.isNaN es6 es3 es6/number/isnan
 Number.isSafeInteger es6 es3 es6/number/issafeinteger
+Number.parseInt es6 es3 es6/number/parseint
 Object.assign es6 es3 es6/object/assign
 Object.entries es8 es3 es6/object/entries
 Object.getOwnPropertyDescriptors es8 es5 es6/object/getownpropertydescriptors