eachMapping(aCallback, aContext, aOrder) {
    throw new Error('Subclasses must implement eachMapping')
  }
Iterate over each mapping between an original source/line/column and a
generated line/column in this source map.

@param Function aCallback
       The function that is called with each mapping.
@param Object aContext
       Optional. If specified, this object will be the value of `this` every
       time that `aCallback` is called.
@param aOrder
       Either `SourceMapConsumer.GENERATED_ORDER` or
       `SourceMapConsumer.ORIGINAL_ORDER`. Specifies whether you want to
       iterate over the mappings sorted by the generated file's line/column
       order or the original's source/line/column order, respectively. Defaults to
       `SourceMapConsumer.GENERATED_ORDER`.