avoid_implementing_value_types false positives

[davidmorgan]

Types that implement something in the dart:html package that descend from Interceptor cause false positives in this lint, because Interceptor has Object-like implementations of operator== and hashCode, making all these classes look like value types to the lint.

e.g. class MockHttpRequest extends Mock implements HttpRequest {} should not violate this lint.

Likely the lint needs to special case Interceptor.