Update test corelib_2/iterable_to_list_test for Dart 2 type inference

According to Leaf, Dart 2 type inference prefers to infer type
which is asked for, even if a more specific type can be inferred
from arguments.

It means that for

  testIterable({"x": 1, "y": 1}.keys, ["x", "y"]);

where

  testIterable(Iterable iterable, List expected, [int depth = 0])

the inferred type of map is Map<String, int>, but inferred type
of list is List (as testIterable argument type is List).

So this test is fixed by specifying type arguments of 'expected'
lists explicitly.

Change-Id: I2deab160038ee7abaad587920cb8fc620e09ebdf
Reviewed-on: https://dart-review.googlesource.com/39441
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

• tests/corelib_2/corelib_2.status[diff]
• tests/corelib_2/iterable_to_list_test.dart[diff]