[tests] Copy comment from corelib_2/unsigned_shift_test to corelib

Accidentally omitted from https://dart-review.googlesource.com/c/sdk/+/186920

TBR=alexmarkov@google.com

Change-Id: I39d3942f411bc9987f4e817bd05932b460ceca2a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/187162
Auto-Submit: Stephen Adams <sra@google.com>
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
diff --git a/tests/corelib/unsigned_shift_test.dart b/tests/corelib/unsigned_shift_test.dart
index e0bf752..c190ff3 100644
--- a/tests/corelib/unsigned_shift_test.dart
+++ b/tests/corelib/unsigned_shift_test.dart
@@ -36,6 +36,9 @@
     //         .   .   .   .   .
     testShift(0x7ffffffffffff000, i);
     testShift(0xfffffffffffff000, i);
+    // Construct the values below to get 'all ones' values on the VM without a
+    // compile-time error for roundned literals on the web. The arithmetic
+    // produces rounded values on the web, so they are effectively testing zero.
     testShift(0x7ffffffffffff000 + 0xfff, i);
     testShift(0xfffffffffffff000 + 0xfff, i);
   }