@0x/contracts-exchange
_assertValidFill()
`@0x/contracts-exchange`: Add `_settleOrder()` unit tests. `@0x/contracts-exchange`: Add explicit tests for `_calculateFillResults()`. `@0x/contracts-exchange`: Add overflow tests to `isolated_fill_order` tests. `@0x/contracts-exchange`: Add explicit `takerAssetFillAmount = 0` test to `isolated_fill_order` tests.