Allow named imports with names `Storage`, `Plugin` and more.

Closure throws an error if it sees a class named `Storage` or `Plugin`.

e.g.: https://closure-compiler.appspot.com/home#code%3D%252F%252F%2520%253D%253DClosureCompiler%253D%253D%250A%252F%252F%2520%2540output_file_name%2520default.js%250A%252F%252F%2520%2540compilation_level%2520SIMPLE_OPTIMIZATIONS%250A%252F%252F%2520%253D%253D%252FClosureCompiler%253D%253D%250A%250Aclass%2520Plugin%257B%257D

This might be fine as one can argue that it is a reserved word. But if these classes are a part of my imports and not global definitions, it should be a clear indication that we can safely mangle this.

## What's the issue?

Closure throws error.

## How do we reproduce the issue?
file-A:
```
export class Plugin {
}
```

file-B:
```
import {Plugin} from './file-A`
const p = new Plugin();
```

Run ☝️ this, with rollup-plugin-closure-compiler.

Start with the file-A and file-B reproduction and run it through rollup-plugin-closure-compiler. Trace how imported names such as Plugin and Storage are handled during compilation, then verify that the reproduction completes without the reported error while preserving the import behavior.

Written by the indexing model from the issue text.