RFR: 6323374: (coll) Optimize Collections.unmodifiable* and synchronized* [v5]

[Ian Graves]

> Modify the `unmodifiable*` methods in `java.util.Collections` to be idempotent. That is, when given an immutable collection from `java.util.ImmutableCollections` or `java.util.Collections`, these methods will return the reference instead of creating a new immutable collection that wraps the existing one.

Ian Graves has updated the pull request incrementally with one additional commit since the last revision:

  Test refactoring. Adding implNote to modified methods

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2596/files
  - new: https://git.openjdk.java.net/jdk/pull/2596/files/ebdd0f85..8109e158

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2596&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2596&range=03-04

  Stats: 185 lines in 2 files changed: 12 ins; 142 del; 31 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2596.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2596/head:pull/2596

PR: https://git.openjdk.java.net/jdk/pull/2596