Unknown type MapEntry in ES6 polyfills

After compiling our very large codebase, we see these warnings:

```
[synthetic:es6/map] :81: WARNING - Bad type annotation. Unknown type MapEntry

[synthetic:es6/map] :85: WARNING - Bad type annotation. Unknown type MapEntry

[synthetic:es6/map] :116: WARNING - Bad type annotation. Unknown type MapEntry
```

We'll have to code in some exceptions, since we fail the build on warning. Presumably this is a bug, rather than something we're doing? Can't see how to put together a test case in the debugger here; [this doesn't work](https://closure-compiler-debugger.appspot.com/#input0%3Dlet%2520m%2520%253D%2520new%2520Map%28%29%253B%250Am.set%28%2522a%2522%252C%2520%2522b%2522%29%253B%250A%26input1%26conformanceConfig%26externs%26refasterjs-template%26includeDefaultExterns%3D1%26CHECK_SYMBOLS%3D1%26CHECK_TYPES%3D1%26CHECK_TYPES_NEW_INFERENCE%3D1%26CLOSURE_PASS%3D1%26LANG_IN_IS_ES6%3D1%26MISSING_PROPERTIES%3D1%26PRESERVE_TYPE_ANNOTATIONS%3D1%26PRETTY_PRINT%3D1%26TRANSPILE%3D1).