Fix ArgumentDeclaration.toString() when no args except rest

[srawlins]

ArgumentDeclaration.toString() unconditionally puts a comma before the rest argument, even if it is the only argument. For example, with the input:

@mixin box-shadow1($shadows...) {
  -webkit-box-shadow: $shadows;
  box-shadow: $shadows;
}

@mixin box-shadow2($arg1, $shadows...) {
  color: $arg1;
  -webkit-box-shadow: $shadows;
  box-shadow: $shadows;
}

@mixin box-shadow2($arg1, $arg2) {
  color: $arg1;
  -webkit-box-shadow: $arg2;
  box-shadow: $arg2;
}

Given this simple visitor and main:

import 'dart:io';

import 'lib/src/ast/sass.dart';
import 'lib/src/value.dart';
import 'lib/src/visitor/interface/statement.dart';

void main(List<String> args) {
  var file = args.first;
  var contents = new File(file).readAsStringSync();
  var sassTree = new Stylesheet.parseScss(contents);
  sassTree.accept(new LittleVisitor());
}

class LittleVisitor implements StatementVisitor<Value> {
  LittleVisitor() {}

  void visitStylesheet(Stylesheet node) {
    for (var child in node.children) {
      child.accept(this);
    }
  }

  void visitIncludeRule(IncludeRule node) {
    stdout.write('  @include ${node.name}');
    stdout.write(node.arguments);
    stdout.write(';\n');
  }

  void visitMixinRule(MixinRule node) {
    stdout.write('@mixin ${node.name}(');
    stdout.write(node.arguments);
    stdout.write(') {\n  ...\n}\n');
  }

  void visitStyleRule(StyleRule node) {
    stdout.write(node.selector);
    stdout.write('{\n');
    for (var child in node.children) {
      child.accept(this);
    }
    stdout.write('}\n');
  }
}

We output the following:

$ dart argument_declaration.dart f.scss
@mixin box-shadow1(, shadows...) {
  ...
}
@mixin box-shadow2(arg1, shadows...) {
  ...
}
@mixin box-shadow2(arg1, arg2) {
  ...
}

This PR fixes the output to look like:

$ dart argument_declaration.dart f.scss
@mixin box-shadow1(shadows...) {
  ...
}
@mixin box-shadow2(arg1, shadows...) {
  ...
}
@mixin box-shadow2(arg1, arg2) {
  ...
}

(the variables do not include $, but that's another issue, if it's an issue at all.)