chore: move core modules into core directory

BREAKING CHANGE:
    This change moves the http module into angular2/, so its import
    path is now angular2/http instead of http/http.

    Many other modules have also been moved around inside of angular2,
    but the public API paths have not changed as of this commit.

modules/angular2/src/core/facade/browser.dart

@@ -0,0 +1,30 @@
+/**
+ * Dart version of browser APIs. This library depends on 'dart:html' and
+ * therefore can only run in the browser.
+ */
+library angular2.src.facade.browser;
+
+import 'dart:js' show context;
+
+export 'dart:html'
+    show
+        document,
+        location,
+        window,
+        Element,
+        Node,
+        MouseEvent,
+        KeyboardEvent,
+        Event,
+        EventTarget,
+        History,
+        Location,
+        EventListener;
+
+final _gc = context['gc'];
+
+void gc() {
+  if (_gc != null) {
+    _gc.apply(const []);
+  }
+}