Change MutableSource so that it does not report unchanged line index when inserting a new line.
Change MutableSource so that it does not report unchanged line index when inserting a new line.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=317175630

google/closure-templates,google/closure-templates,yext/closure-templates,google/closure-templates,google/closure-templates,yext/closure-templates,yext/closure-templates,yext/closure-templates,google/closure-templates