Move new tests from tests/isolate to tests/lib_2/isolate.

Fix test expectations.
Improve test (overly complex).
Mark tests/isolate as immutable to the presubmit script (like other migrated Dart 1 test directories).

(Gardening)

Change-Id: Iec59c59ca992b3dacafbf82176e4e8bd8c5bd3c5
Reviewed-on: https://dart-review.googlesource.com/42945
Reviewed-by: William Hesse <whesse@google.com>

PRESUBMIT.py

@@ -115,6 +115,7 @@ def _CheckNewTests(input_api, output_api):
       ("tests/corelib/",        "tests/corelib_2/"),
       ("tests/lib/",            "tests/lib_2/"),
       ("tests/html/",           "tests/lib_2/html/"),
+      ("tests/isolate/",        "tests/lib_2/isolate/")
   ]
 
   result = []