Supress Warnings of the Java compiler where it is safe to do so

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Java code like src/library/scala/collection/concurrent/MainNode.java creates warnings every time we compile Scala.

Figure out whether it is safe to add things like {{{@SuppressWarnings("rawtypes")}}} and {{{@SuppressWarnings("unchecked")}}}.

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Imported From: https://issues.scala-lang.org/browse/SI-7484?orig=1
Reporter: @soc
Affected Versions: 2.11.0-M2

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@adriaanm said:
scala/scala#2539