number.toFixed causes "Error: don't know how to turn this value into a node"

**Describe the bug**

I wrote this sample code:

```ts
export function formatCurrency(amount: number, currencyCode: string): string {
let formattedAmount = amount.toFixed(2);

switch (currencyCode) {
case 'EUR':
return `€${formattedAmount}`;
case 'USD':
return `$${formattedAmount}`;
default:
return `${formattedAmount} ${currencyCode}`;
}
}
```

Tried to compile the app, and got the error "Error: don't know how to turn this value into a node". On digging a little deeper, I was able to trace the bug to this package. The package apparently tried to inline `toFixed` and failed.

**Configuration**

How are you using babel-minify?

Used indirectly through `@storybook/core@5.1.9`

No repository file or test is named. Reproduce the failure with the TypeScript sample and the babel-minify version used indirectly by @storybook/core@5.1.9, then trace the toFixed inlining path. Done means the sample compiles without the node-conversion error and a regression test covers amount.toFixed(2).

Written by the indexing model from the issue text.