function getCompilerFlags(mode) {
  return mode == 'debug' ?
      mergeObjects(config.COMPILER_FLAGS_COMMON, config.COMPILER_FLAGS_DEBUG) :
      mergeObjects(config.COMPILER_FLAGS_COMMON, config.COMPILER_FLAGS_OPT);
}
@param {string} mode One of "debug" or "opt".
@return {!Object} An object holding all compiler flags and their values.