[DDC] Add context.enterNode call to "if" in "else" clause
[DDC] Add context.enterNode call to "if" in "else" clause

Without this there is no way to get source maps on the else if in e.g.
```
    if (core.StateError.is(e$)) {
      core.print("StateError");
    } else if (core.ArgumentError.is(e$)) {
      let e = e$;
      {
        core.print(dart.str`ArgumentError: ${e}`);
      }
    }
```

Bug:
Change-Id: I0203fe7f1e0c14c677e16d61e7649624b72d41cd
Reviewed-on: https://dart-review.googlesource.com/23673
Reviewed-by: Vijay Menon <671ecbb18d31a1c038875027dcfe976af959ea9f@google.com>
Commit-Queue: Jens Johansen <05223a3c6bdf76af6f84755eae231abeebcf4530@google.com>

dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk