Cannot build identifier with TypeScript typeAnnotation

Trying to build an identifier with a TypeScript typeAnnotation throws an error because the expected type of the type annotation is `TypeAnnotation | null`, as defined in the flow defs https://github.com/benjamn/ast-types/blob/f4a7c6db8e0eed1f25971d4fcbb84843c83ad615/def/flow.js#L209-L210

For example, if I parse `const x: string` and look at the AST for the identifier I see

```
{
"type": "Identifier",
"name": "x",
"typeAnnotation": {
"type": "TSTypeAnnotation",
"typeAnnotation": {
"type": "TSStringKeyword"
}
}
}
```

But if I try to create this with a builder like so

```
astTypes.builders.identifier.from({ name: 'x', typeAnnotation: astTypes.builders.tsTypeAnnotation(astTypes.builders.tsStringKeyword()) })
```

I get the error

```
Uncaught Error: {typeAnnotation: [object Object], loc: null, type: TSTypeAnnotation, comments: null} does not match field "typeAnnotation": TypeAnnotation | null of type Identifier
```

I'll work on a fix for this and submit a PR

Start with def/flow.js at the referenced lines, then compare the Identifier field definition with the TSTypeAnnotation produced by the shown builder call. Reproduce the failure using the provided const x: string example. Done means the TypeScript annotation can be supplied to the identifier builder without the type-mismatch error.

Written by the indexing model from the issue text.