Code formatter: syntax error after formating heredoc with line break before terminator

[amatalai]

Environment

• Elixir & Erlang versions: Elixir 1.6.0-dev (7b8d5ec28) (compiled with OTP 20)
• Operating system: macOS 10.13 (17A365) - High Sierra

Current behavior

Formatter output for this file

defmodule Example do
  @moduledoc false
  def text do
    """
    heredoc \
    """
  end
end

looks like this

defmodule Example do
  @moduledoc false
  def text do
    """
    heredoc """
  end
end

which ends as ** (SyntaxError) lib/example.ex:5: invalid location for heredoc terminator, please escape token or move it to its own line: """

Expected behavior

VERSION A

Don't remove line break from last line of heredoc

VERSION B

Don't allow to use line break in last line of heredoc like some other languages. In this case I would like to make PR, something like this: https://github.com/amatalai/elixir/blob/7817b634a9877368ac314b55aa4a8ab53e881cc4/lib/elixir/src/elixir_tokenizer.erl#L725-L727, but maybe with better message :)

[josevalim]

I will fix the formatter accordingly. Thank you!

[amatalai]

💚