Hack to prevent introducing variables starting with "_" into global scope when inlining functions.

R=acleung
DELTA=30 (28 added, 0 deleted, 2 changed)


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git-svn-id: http://closure-compiler.googlecode.com/svn/trunk@2490 b0f006be-c8cd-11de-a2e8-8d36a3108c74