Fix for https://github.com/mozilla/source-map/issues/142

FIxing an issue where instanceOf isn't matching when the SourceNodes come from two different source-map instances.
At the top of this module can you define:

    // Private symbol for identifying `SourceNode`s when multiple versions of
    // the source-map library are loaded. This MUST NOT CHANGE across
    // versions!
    var isSourceNode = "$$$isSourceNode$$$";

And then use it elsewhere like `aChunk[isSourceNode]` and `this[isSourceNode] = true` etc.

Thanks!