Add property to File to extract extension

[zanderso]

Add a property to File (or Uri?) for extracting the file extension, e.g. "file.txt" -> ".txt", "program.exe" -> ".exe", "noext" -> "". Similar to pythons os.path.splitext: https://docs.python.org/2/library/os.path.html#os.path.splitext

[DartBot]

This comment was originally written by @zoechi

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It's in the path package
http://www.dartdocs.org/documentation/path/1.3.0/index.html#path/path@id_extension

[zanderso]

Leaving Area-Library, and leaving open here and filing on api.dartlang.org as well because this is not discoverable and is a poor user experience.

[natebosch]

Would it be sufficient to mention the path package in the docs for File?