introduce web_components pkg for consolidated polyfills

pkg/pkg.status

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # Don't run any test-like files that show up in packages directories. It
 # shouldn't be necessary to run "pub install" in these packages, but if you do
 # it shouldn't break the tests.
 */packages/*/*: Skip
 */*/packages/*/*: Skip
 */*/*/packages/*/*: Skip
@@ skipping 270 matching lines @@
 polymer/example/canonicalization/test/deploy3_test: Fail, OK # tests deploy only behavior
 polymer/example/canonicalization/test/dir/deploy_test: Fail, OK # tests deploy only behavior
 polymer/example/canonicalization/test/dir/deploy2_test: Fail, OK # tests deploy only behavior
 mutation_observer/test/mutation_observer_test: Skip # 16679 - Crashes or times out after 33 roll.
 
 [ $compiler == none && ( $runtime == dartium || $runtime == drt ) && $checked ]
 polymer/test/custom_event_test: Skip # http://dartbug.com/15517
 
 # Skip tests on the VM if the package depends on dart:html
 [ $runtime == vm ]
-custom_element: Skip
 template_binding: Skip
 mutation_observer: Skip
 polymer_expressions/test/syntax_test: Skip
 polymer_expressions/test/globals_test: Skip
 polymer_expressions/test/bindings_test: Skip
 third_party/html5lib/test/browser/browser_test: Skip
 
 [ $runtime == safari || $runtime == ie9 || $runtime == ie10 ]
 polymer_expressions/test/globals_test: Fail # Issue 16568
 
@@ skipping 49 matching lines @@
 collection/test/algorithms_test: PubGetError
 collection/test/iterable_zip_test: PubGetError
 collection/test/priority_queue_test: PubGetError
 collection/test/unmodifiable_collection_test: PubGetError
 collection/test/wrapper_test: PubGetError
 stack_trace/test/chain_test: PubGetError
 stack_trace/test/frame_test: PubGetError
 stack_trace/test/trace_test: PubGetError
 stack_trace/test/vm_test: PubGetError