RFR: JDK-8283480: Make AbstractStringBuilder sealed

[Joe Darcy]

As part of updating the core libraries to be sealed, the package-access AbstractStringBuilder, implementation superclass of StringBuilder and StringBuffer, can be marked as sealed with those two subclasses on its permits list.

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Commit messages:
 - Merge branch 'master' into JDK-8283480
 - Fix whitespace.
 - JDK-8283480: Make AbstractStringBuilder sealed

Changes: https://git.openjdk.java.net/jdk/pull/7898/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7898&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283480
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7898.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7898/head:pull/7898

PR: https://git.openjdk.java.net/jdk/pull/7898