feat(components): implement css encapsulation for transcluding components

* Extract ComponentCssLoader so it can be shared by ShadowDomComponentFactory and TranscludingComponentFactory
* Provide a built-in implementation of WebPlatformShim using css_shim
* Implement a CSS transformer similar to the one provided by Platform.JS
* Rename EventHandler into ShadowBoundary

Author: vsavkin

lib/core_dom/component_css_loader.dart

@@ -0,0 +1,69 @@
+part of angular.core.dom_internal;
+
+class ComponentCssLoader {
+  final Http http;
+  final TemplateCache templateCache;
+  final WebPlatformShim platformShim;
+  final ComponentCssRewriter componentCssRewriter;
+  final dom.NodeTreeSanitizer treeSanitizer;
+  final Map<_ComponentAssetKey, async.Future<dom.StyleElement>> styleElementCache;
+
+  ComponentCssLoader(this.http, this.templateCache, this.platformShim,
+                      this.componentCssRewriter, this.treeSanitizer,
+                      this.styleElementCache);
+
+  async.Future<List<dom.StyleElement>> call(String tag, List<String> cssUrls) =>
+      async.Future.wait(cssUrls.map((url) => _styleElement(tag, url)));
+
+  async.Future<dom.StyleElement> _styleElement(String tag, String cssUrl) {
+    final element = styleElementCache.putIfAbsent(
+        new _ComponentAssetKey(tag, cssUrl),
+        () => _loadNewCss(tag, cssUrl));
+    return element;
+  }
+
+  async.Future _loadNewCss(String tag, String cssUrl) {
+    return _fetch(cssUrl)
+        .then((css) => _shim(css, tag, cssUrl))
+        .then(_buildStyleElement);
+  }
+
+  async.Future<String> _fetch(String cssUrl) {
+    return http.get(cssUrl, cache: templateCache)
+        .then((resp) => resp.responseText, onError: (e) => '/* $e */');
+  }
+
+  String _shim(String css, String tag, String cssUrl) {
+    final shimmed = platformShim.shimCss(css, selector: tag, cssUrl: cssUrl);
+    return componentCssRewriter(shimmed, selector: tag, cssUrl: cssUrl);
+  }
+
+  dom.StyleElement _buildStyleElement(String css) {
+    var styleElement = new dom.StyleElement()..appendText(css);
+    treeSanitizer.sanitizeTree(styleElement);
+    return styleElement;
+  }
+}
+
+class _ComponentAssetKey {
+  final String tag;
+  final String assetUrl;
+
+  final String _key;
+
+  _ComponentAssetKey(String tag, String assetUrl)
+      : _key = "$tag|$assetUrl",
+        this.tag = tag,
+        this.assetUrl = assetUrl;
+
+  @override
+  String toString() => _key;
+
+  @override
+  int get hashCode => _key.hashCode;
+
+  bool operator ==(key) =>
+      key is _ComponentAssetKey
+      && tag == key.tag
+      && assetUrl == key.assetUrl;
+}