Update dart2js

"1" is not a valid bash variable name (so follow_link() doesn't work, for example, on OS/X-10.8*, resulting in the following type of output:
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$ dart2js demo.dart
/usr/local/bin/dart2js: line 9: 1=/usr/local/dart/dart-sdk/bin/dart2js: No such file or directory
/usr/local/bin/dart2js: line 9: 1=/usr/local/dart/dart-sdk/bin/dart2js: No such file or directory
...
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simply convert positional to a valid name, and dart2js works.