Make document.getElementsByTagname return a NodeList; consolidate element collection to collectElement; fix #1744
De-duplicate reftest code
reftest: Allow passing Servo args after --
Implement `position: absolute` for replaced elements.

Add reftests for replaced and nested absolute flows.
Reftest both CPU and GPU rendering

Fixes #1179.
When calling constructors, retrieve the global object from the callee rather than the JSContext.
Return a JS<Window> from global_object_for_{dom,js}_object.
Merge the Wrap and Wrap_ functions in codegen.
Stop passing the scope argument to global object bindings' Wrap functions.
Pass &JS<Window> to some constructors.
Pass &JS<Window> to reflect_dom_object.
Pass &JS<Window> to the Wrap functions in codegen.
Get the Page directly off the Window in CreateBindingJSObject.
Get the Page from the global object for ResolveProperty.
auto merge of #1809 : Ms2ger/servo/constructor-global-from-object, r=jdm

...rather than the JSContext.

Requires <https://github.com/mozilla-servo/rust-mozjs/pull/51>.
Introduce a test IDL file.
Reorder implementations in conversions.rs.
Extend and update primitive conversions.
Use JSValConvertible::to_jsval from codegen.
Implement support for returning nullable primitive types.