Lint to check that assert does not do assignment

[dnfield]

BAD:

assert(foo = true); // Always evaluates to true, and will always pass.
assert(bar = false); // Always evaluates to false, and will always fail.

GOOD:

assert(foo == true);
assert(bar == false);

// Intentionally set foo to true when asserts are enabled.
assert(() {
  foo = true;
  return true;
}());

This has definitely caused problems for Flutter before - maybe @jonahwilliams remembers what, but I seem to remember him finding an assertion written like this that, when corrected, failed so often it had to just be disabled.

[dnfield]

Probably should be a seperate bug, but a similar lint would be good for if (foo = true), while (foo = true), when the literals true or false are used.