Fix mouseenter and mouseleave for JsAction.

It's unclear how mouseenter/mouseleave ever worked with the existing code, but reports (and initial testing) suggest it did...
	Change on 2014/05/27 by dfli <dfli@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68022600
Add a jsaction.event.getTarget() function and use it in several
places in jsaction/event.js.

A future CL will use this function in several places in Tactile code
to help make Tactile code run in IE8.
	Change on 2014/05/27 by eparker <eparker@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68044672
JsAction: remove goog.array dependency from event contract binaries.
	Change on 2014/06/02 by ruilopes <ruilopes@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68380014
Improve 'mouseenter' and 'mouseleave' handling when the mouse enters/exits the window.

Also fix jsaction.dom.contains to actually check node equivalence to handle edge cases where mouseenter / mouseleave fail to register.
	Change on 2014/06/10 by dfli <dfli@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=68945802