Updates for next sdk

Review URL: https://chromiumcodereview.appspot.com//11780004
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