RFR: 8278268 - (ch) InputStream returned by Channels.newInputStream should have fast path for FileChannel targets [v20]

[Markus KARG]

> This sub-issue defines the work to be done to implement JDK-8265891 solely for the particular case of FileChannel.transferFrom(ReadableByteChannel), including special treatment of SelectableByteChannel.

Markus KARG has updated the pull request incrementally with two additional commits since the last revision:

 - Abstracted assertStreamContents
 - Abstracted assertNullPointerException

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/6711/files
  - new: https://git.openjdk.org/jdk/pull/6711/files/50146a9d..6d3e1835

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=6711&range=19
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=6711&range=18-19

  Stats: 114 lines in 3 files changed: 48 ins; 62 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/6711.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/6711/head:pull/6711

PR: https://git.openjdk.org/jdk/pull/6711