Intern literals based on (estimated) profitability.

Literals are only interned if considered profitable taking into
account occurrences and source size.

Reverts the small regression introduced by the new literal interener
and additionally reduces ~0.5% the size of compressed JavaScript.

Change-Id: I2594cc1a1c99ee92d41f8741ea80558dc781c970
Simplifies FileUpload

Change-Id: I78c94c1ebbb589c3c29a950d2507dec82aea2c66
Review-Link: https://gwt-review.googlesource.com/#/c/6230/
Removes Opera from c.g.gwt.user.client.impl.DOMImpl

Change-Id: Ib04aec4aee5eb441e29f25811464a4023d9bebf7
Removes Opera from remaining c.g.gwt.user.client

Change-Id: I3d1c3e9e1963ec80f4280606ad5e0843776650b0
Merge remote-tracking branch 'upstream/master'