This reverts commit 3ddd311.

For example, `ref` not `_ref`. It's cleaner.

This also fixes #3816.

Name generated variables without leading underscore

Fix #3822: Include delimiters in string/regex locations

The following two lines might seem equivalent:

    for n in [1, 2, 3] by  a then a = 4; n
    for n in [1, 2, 3] by +a then a = 4; n

But they used not to be, because `+a` was cached into a `ref`, while the plain
`a` wasn’t. Now even simple identifiers are cached, making the two lines
equivalent as expected.

Fix #3778: Make for loops more consistent

As discussed in #3039 (comment).
This is the first step to implement dynamic object literal keys (see #3597).

This also fixes #1392.

In short, `super` is now allowed:

    # in class definitions:
    class A
      instanceMethod: -> super
      @staticMethod: -> super
      @staticMethod2 = -> super

    # in assignment where the next to last access is 'prototype':
    A::m = -> super
    A.prototype.m = -> super
    a.b()[5]::m = -> super
    A::[x()] = -> super
    class B
      @::m = -> super

Allow super in methods with dynamic names

Improve error messages for unexpected regexes

The third element in a token should just be an object containing line
number and column info. This PR fixes the problem with one of the tokens
being set incorrectly.

Fix incorrect token representation

- Invalid `\x` and `\u` escapes now throw errors.
- U+2028 and U+2029 (which JavaScript treats as newline characters) are now
  escaped to `\u2028` and `\u2029`, respectively.
- Octal escapes are now forbidden not only in strings, but in regexes as well.
- `\0` escapes are now escaped if needed (so that they do not form an octal
  literal by mistake). Note that `\01` is an octal escape in a regex, while `\1`
  is a backreference. (Added a test for backreferences while at it.)
- Fixed a bug where newlines in strings weren't removed if preceded by an
  escaped character.

Fix #3795: Never generate invalid strings and regexes

- Erraneous tokens are now fully underlined with ^:s.
- The error messages are now a bit more consistent.

Improve lexer error messages

Replace `last array` helper with `[..., last] = array`

Fixed copyright range to be updated to 2015

The following is now allowed:

    o =
      a: 1
      b: 2
      "#{'c'}": 3
      "#{'d'}": 4
      e: 5
      "#{'f'}": 6
      g: 7

It compiles to:

    o = (
      obj = {
        a: 1,
        b: 2
      },
      obj["" + 'c'] = 3,
      obj["" + 'd'] = 4,
      obj.e = 5,
      obj["" + 'f'] = 6,
      obj.g = 7,
      obj
    );

- Closes #3039. Empty interpolations in object keys are now _supposed_ to be
  allowed.
- Closes #1131. No need to improve error messages for attempted key
  interpolation anymore.
- Implementing this required fixing the following bug: `("" + a): 1` used to
  error out on the colon, saying "unexpected colon". But really, it is the
  attempted object key that is unexpected. Now the error is on the opening
  parenthesis instead.
- However, the above fix broke some error message tests for regexes. The easiest
  way to fix this was to make a seemingly unrelated change: The error messages
  for unexpected identifiers, numbers, strings and regexes now say for example
  'unexpected string' instead of 'unexpected """some #{really long} string"""'.
  In other words, the tag _name_ is used instead of the tag _value_.
  This was way easier to implement, and is more helpful to the user. Using the
  tag value is good for operators, reserved words and the like, but not for
  tokens which can contain any text. For example, 'unexpected identifier' is
  better than 'unexpected expected' (if a variable called 'expected' was used
  erraneously).
- While writing tests for the above point I found a few minor bugs with string
  locations which have been fixed.

Fix #3597: Allow interpolations in object keys

use more CoffeeScript syntax

Allow multiline comment at end of an object definition [Fixes #3761]

Now the same hack as for reserved identifier tokens in the lexer is used
instead.

Fix #3846: Fix odd start token of implicit objects

Fix error message for invalid escape at end of regex

Prevent writing the same file several times (fixes #3753)

fixed being unable to use 'yield throw'

added a lot of ES6 generator tests

fixed overly fragile repl test to work with 0.12

…e last line

fixed yield return producing incorrect output when used outside of the last line

Fix single-line heredocs starting with "undefined"