RFR: 8275302: unexpected compiler error: cast, intersection types and sealed

[Vicente Romero]

Please review this fix which is syncing the compiler with the sealed classes spec. Basically the compiler is failing to accept as valid a cast when the type to be cast is an intersection type, even when all the checks mandated by the spec has been done.

TIA

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Commit messages:
 - 8275302: unexpected compiler error: cast, intersection types and sealed

Changes: https://git.openjdk.java.net/jdk/pull/6009/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6009&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275302
  Stats: 14 lines in 2 files changed: 13 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6009.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6009/head:pull/6009

PR: https://git.openjdk.java.net/jdk/pull/6009