static transform(tempVarTransformer, tree) {
    tree = alphaRenameThisAndArguments(tempVarTransformer, tree);
    let body = convertConciseBody(tree.body);
    return new FunctionExpression(tree.location, null, tree.functionKind,
        tree.parameterList, null, [], body);
  }
Shallowly transforms |tree| into a FunctionExpression and adds the needed
temp variables to the |tempVarTransformer|.
@param {TempVarTransformer} tempVarTransformer
@param {ArrowFunction} tree
@return {FunctionExpression}