Implement `position: absolute` for replaced elements.

Add reftests for replaced and nested absolute flows.
Children of fixed flows make into parent DL

  fixes #1831
auto merge of #1805 : pradeep90/servo/replaced-absolute-position, r=pcwalton

Add reftests for replaced and nested absolute flows.
auto merge of #1832 : ibnc/servo/fixed_display_lists_fixed, r=larsbergstrom

  fixes #1831
Remove Node.hasAttributes() method that was removed from the specification.
auto merge of #1835 : Ms2ger/servo/hasAttributes, r=metajack
Add some tests for string codegen.
Add tests for optional nullable arguments without default values.
Add interface codegen tests.
auto merge of #1839 : Ms2ger/servo/optional-test, r=jdm
Add disabled tests for optional nullable arguments with non-null default values.
Inline dataLoc in the one place that can see it and the one place that can't.
Avoid a type error with a non-null default value for an optional nullable primitive argument.
Avoid a type error with a non-null default value for an optional nullable string argument.
auto merge of #1846 : Ms2ger/servo/non-null-default, r=jdm