Ensure dart_analyzer is built to run on a 1.6+ VM.

Ensure dart_analyzer is built to run on a 1.6+ VM.
Committed: https://code.google.com/p/dart/source/detail?r=16147

[devoncarew, 2012-12-14 00:14:32]

Somehow, the buildbots are creating a dart_analyzer.jar that won't work on a 1.6
VM. The class file versions are 51.0. This CL ensures that we write out 1.6
class files.

[scheglov, 2012-12-14 01:18:02]

LGTM

[danrubel, 2012-12-14 01:49:13]

lgtm