BIND 10 #2375: implement MasterLexer getNextToken() and ungetToken()

[BIND 10 Development]

#2375: implement MasterLexer getNextToken() and ungetToken()
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                   Reporter:         |                 Owner:  vorner
  jinmei                             |                Status:  reviewing
                       Type:  task   |             Milestone:
                   Priority:         |  Sprint-20121120
  medium                             |            Resolution:
                  Component:         |             Sensitive:  0
  libdns++                           |           Sub-Project:  DNS
                   Keywords:         |  Estimated Difficulty:  5
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
  loadzone-ng                        |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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Comment (by jinmei):

 one more thing: we don't want to propagate things defined in the
 master_lexer_internal class in general, much less anything related to
 `InputSource`.  In that sense we cannot propagate
 `InputSource::ReadError` as a result of doing nothing for it in
 `getNextToken()`.  I suggest defining it within master_lexer.h,
 whether it's define in the namespace scope of isc::dns or as
 part of the `MasterLexer` class.

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Ticket URL: <http://bind10.isc.org/ticket/2375#comment:11>
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