Fix AssertionError when running examples/identity on underscore.js

[benjamn]

Using Underscore version 1.4.2:
http://underscorejs.org/underscore.js

Stack trace:

assert.js:102
  throw new assert.AssertionError({
        ^
AssertionError: false == true
    at pushSlice (/Users/benjamn/node_modules/recast/lib/patcher.js:34:20)
    at Patcher.self.get (/Users/benjamn/node_modules/recast/lib/patcher.js:50:9)
    at self.getReprinter (/Users/benjamn/node_modules/recast/lib/parser.js:63:28)
    at print (/Users/benjamn/node_modules/recast/lib/printer.js:42:20)
    at self.getReprinter (/Users/benjamn/node_modules/recast/lib/parser.js:59:21)
    at Array.forEach (native)
    at self.getReprinter (/Users/benjamn/node_modules/recast/lib/parser.js:55:22)
    at print (/Users/benjamn/node_modules/recast/lib/printer.js:42:20)
    at self.getReprinter (/Users/benjamn/node_modules/recast/lib/parser.js:59:21)
    at Array.forEach (native)

[benjamn]

Seems to be related to the way source locations are assigned to regular expression literals.