Avoid duplicate work on .plus() by not re-linking all the bindings (or scanning them to determine if they need re-linking, etc.)

This has very bad outcomes in graph configurations with large numbers of bindings in the root graph, and lots of repeated calls to .plus().
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=56842169
Fix checkstyle violation - extra import.