js_mock_test currently tests mocks on native types by defining
the native types using JS interop. This is error-prone, as the
underlying native type has different type semantics than JS
interop types. Also moves these tests to lib/js and adds some
additional checks.
Change-Id: If4bba54d589a38d7f7fbf1c26c6c6f755ec6b9ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/173361
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Srujan Gaddam