function getLexicalBindingNames(tree) {
  let names = new StringSet();
  if (tree !== null && tree.type === VARIABLE_DECLARATION_LIST &&
      tree.declarationType !== VAR) {
    let visitor = new FindNames(names);
    for (let i = 0; i < tree.declarations.length; i++) {
      visitor.visitAny(tree.declarations[i].lvalue);
    }
  }
  return names;
}
Gets the variable names in a declaration list. This is used to get the names
from for loops (for, for-in and for-of)
@param {VariableDeclarationList} tree
@return {StringSet} An object as a map where the keys are the variable names.