Updates for next sdk

Review URL: https://chromiumcodereview.appspot.com//11780004
build method returns List<Future> instead of void
fix whitespace in components
need to use addRaw for the initial triple quote.
After this fix, the code for empty component looks like:

    _root.innerHtml = '''''';

Review URL: https://chromiumcodereview.appspot.com//11781006
missing element tag defaults to span, per the spec

Review URL: https://chromiumcodereview.appspot.com//11636060
Change backend to use new output.

Review URL: https://chromiumcodereview.appspot.com//11673005
This change introduces:

 * Benchmark and AsyncBenchmark base classes: the former is practically BenchmarkBase from benchmark_harness with some modifications. Eventually we should unify the two.

 * perf utilities: a perf test runner that runs our tests in DRT (dart + js) and writes results in a json file, an update utility script to update results, and a comparison script that shows the difference between two sets of results.

 * 25 benchmarks

Review URL: https://chromiumcodereview.appspot.com//11683002
Merge pull request #296 from selkhateeb/build-fix

build method returns List<Future> instead of void
work around issue in new unittest breaking compact_vm_config
our tests weren't termining with the current unittest pkg

Review URL: https://chromiumcodereview.appspot.com//11820042
fix a checked mode bug and work around vm parse issue
also run our DRT tests in checked mode now

BUG=

Review URL: https://chromiumcodereview.appspot.com//11884009
Fix #155 - allows users to omit the top-level script tag if it is basically
empty. This makes it possible to treat any HTML file as an entrypoint.

If a file doesn't have a script tag, we assume it to be:
   <script type='application/dart'>main() {}</script>

We also provide a --components_only flag, that can be used to avoid generating
the entrypoint and bootstrap logic, and generate only the components defined in
a file.

Review URL: https://chromiumcodereview.appspot.com//11699003
update changelog to include notes about 0.2.11 and upcoming changes
Updates to the web_ui package to use lib v2

Review URL: https://chromiumcodereview.appspot.com//11855002
Fix #312 - use the correct import location in the bootstrap code.

Review URL: https://chromiumcodereview.appspot.com//11918002
Add a warning about using a component that isn't referenced.
Warning was only being issued for is="", now it is for x- tags too. Also added tests for new and existing warnings.

Review URL: https://chromiumcodereview.appspot.com//12016007
add support for a --full command-line option

Add support for a --full command-line option. This indicates that a full build is requested.
WebComponent: implement Node.nodeValue

Closes https://github.com/dart-lang/web-ui/issues/320
Use correct Path ctor

Closes https://github.com/dart-lang/web-ui/issues/322
Merge pull request #321 from kevmoo/master

Fixes for libv2 changes
Fixes to have a clean package after the latest breaking changes from bleeding
edge.

BUG=

Review URL: https://chromiumcodereview.appspot.com//12052035
Update .gitignore