Strict null checks for Map members

TypeScript Version: 2.0.0-beta
Code

export interface INode {
    type: string;
    parentNode?: INode;
}

export interface IIdentifierNode extends INode {
    name: string;
}

public static isIdentifierNode (node: INode): node is IIdentifierNode {
    return node.type === NodeType.Identifier;
}

var namesMap = new Map<string, string>();

// main part
if (Nodes.isIdentifierNode(node) && namesMap.has(node.name)) {
    node.name = namesMap.get(node.name);  //`Type 'string | undefined' is not assignable to type 'string'.`
}

Expected behavior:
No errors

Actual behavior:
Type 'string | undefined' is not assignable to type 'string'.

Looks like here no TypeGuards for Maps?

No file or test is named. Start by reproducing the TypeScript 2.0 strict-null-check example with Map.has and Map.get, then trace the type-checking behavior for Map members; done means the shown assignment produces no error while preserving correct undefined handling.