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Creates a FileReader that reads files from memory. The manifest parameter is the content of the manifest file. The baseFileReader parameter is an optional FileReader that will be used to read files that are not the manifest. If baseFileReader is not specified, then an error will be returned when a file other than the manifest is read.
Return
A FileReader that reads files from memory.
Parameters
manifest
The content of the manifest file.
base
An optional FileReader that will be used to read files that are not the manifest.