Calculates coverage statistics for the current source file.
function jscoverage_recalculateSourceTab() {
  if (! jscoverage_currentFile) {
    jscoverage_endLengthyOperation();
    return;
  }
  var progressLabel = document.getElementById('progressLabel');
  progressLabel.innerHTML = 'Calculating coverage ...';
  var progressBar = document.getElementById('progressBar');
  ProgressBar.setPercentage(progressBar, 20);
  setTimeout(jscoverage_makeTable, 0);
}