Ensure that instrumentation works with functions that return coroutines.

[jmalloc]

That is:

use Generator as Coroutine;

function a() : Coroutine
{
   yield 1;
}

/**
 * This function is annotated as coroutine, but is not actually a generator,
 * which is probably tripping up the stack trace generator.
 */
function b() : Coroutine
{
    return a();
}

FYI @ezzatron

[jmalloc]

The problem is that the tracing code injections a yield statement, changing this non-generator to a generator. This may be a little involved to fix properly, as the instrumentor will need to travserse the AST to look for existing yield statements before outputting the start of the instrumentation code.

As a workaround, simply annotate the outer function's return value as Generator instead of Coroutine.