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The actual values assigned to tokens was inherited from the yacc-based
grammar. With the most recent cleanups, all single-char tokens such as
commas, semis, parens, etc., that get returned from lexer.next simply
as their Unicode values are below utf8.RuneSelf (i.e., 7bit ASCII).
Lower the initial starting value for named token constants accordingly.

Change-Id: I7eb8e584dbb3bc7f9dab849d1b68a91320cffebd
Reviewed-on: https://go-review.googlesource.com/19913

Reviewed-by: Matthew Dempsky <mdempsky@google.com>