Add optional separator argument to StringBuffer.addAll()

[munificent]

Consider:

writeFunctionCall(StringBuffer buffer, String name, Iterable args) {
  buffer.write(name);
  buffer.write("(");
  buffer.write(args.join(", ")); // <--
  buffer.write(")");
}

On the marked line, we're building up an intermediate string only to add it back to the StringBuffer. That's some wasted allocations.

Maybe a cleaner solution is to add an optional separator argument to StringBuffer.writeAll():

writeFunctionCall(StringBuffer buffer, String name, Iterable args) {
  buffer.write(name);
  buffer.write("(");
  buffer.writeAll(args, ", ");
  buffer.write(")");
}

[lrhn]

Added Duplicate label.
Marked as being merged into #8694.

[munificent]

Well, that will teach me to file bugs without searching first!