support layout tests

[sigmundch]

Add support in test.dart for running layout tests.

Bill - I was thinking of the following strategy to address this issue and issue #3864, but I wanted to double check with you first.

I'm thinking of adding a way to specify flags for the drt binary. We could specify them from the test like we do for VMOptions and DartcOptions (comments in the actual test). These options would be passed as flags to drt-trampoline.

To address layout tests we could do:
// DRTOptions= --expected_png_output=test_expectation_foo.png

To support shadow dom tests (issue #3864) we would pass:
// DRTOptions= --enable_shadow_dom
(or something similar)

Thoughts?

[sethladd]

Added Triaged label.

[whesse]

I'm thinking it would be more localized if the flags could be created inside drt-trampoline. I'm not sure if we need these just on Dartium, or also on dart2ds on drt.
Rather than add additional options to be read by the test scripts, and passed on to drt-trampoline, why not just let drt-trampoline decide, based on comments in the test, what options to add?

I'm surprised that we want to add options to each and every test file, if there is a general pattern to the options, that we could create automatically.

If we do want this in the tests, could we consolidate all of these option types to
// RuntimeOptions(vm)=
// RuntimeOptions(drt)=
// CompilerOptions(dartc)=

---

Removed Type-Defect label.
Added Type-Enhancement label.

[sigmundch]

In the case of --enable_shadow_dom, I think you are right, we should just do this directly in drt_trampoline/run_selenium.

Sam was playing with some of this today and it seems that drt has the shadowDom enabled by default, so we just had to add the flag to dartium in run_selenium.

As for layout tests. We will need the test scripts to do something special because we don't have to use the browser-test-wrapping that we normally do. I'll brainstorm a bit more here and follow up with more ideas.

[sigmundch]

We added support for layout tests long ago, but recently we discovered better ways to tests shadow dom that do not require layout tests. So this feature was later removed.

---

Added Fixed label.