RFR: 8232765: NullPointerException at Types.eraseNotNeeded() when compiling a class [v3]

[Guoxiong Li]

> Hi all,
> 
> The method `types::asSuper` may return `null` so that `types.erasure(types.asSuper(iterator.type.getReturnType(), syms.iteratorType.tsym))` throws `NPE`.
> 
> This patch sets the return type and outputs the corresponding error message if `types::asSuper` returns `null`.
> 
> Thank you for taking the time to review.
> 
> Best Regards.

Guoxiong Li has updated the pull request incrementally with one additional commit since the last revision:

  Revise error message. Add test cases. Add comment.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2099/files
  - new: https://git.openjdk.java.net/jdk/pull/2099/files/cbbaeddc..415e0efa

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2099&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2099&range=01-02

  Stats: 245 lines in 4 files changed: 191 ins; 46 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2099.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2099/head:pull/2099

PR: https://git.openjdk.java.net/jdk/pull/2099