[code-reflection] RFR: Fix compilation issue with CodeElement.elements

[Paul Sandoz]

Code copied into the jdk.compiler module cannot compile against new APIs added in the current release.

Also, correct result type of `VarStoreOp`.

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Commit messages:
 - Fix comilation issue with CodeElement.elements.

Changes: https://git.openjdk.org/babylon/pull/28/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=28&range=00
  Stats: 18 lines in 3 files changed: 9 ins; 1 del; 8 mod
  Patch: https://git.openjdk.org/babylon/pull/28.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/28/head:pull/28

PR: https://git.openjdk.org/babylon/pull/28