Refactor of PolymerExpressions. Adds "as" expressions.

pkg/pkg.status

Index: pkg/pkg.status
diff --git a/pkg/pkg.status b/pkg/pkg.status
index f9f324435f14d959ffd9b8c686befc7b207449ac..acc06e2d8ab3a6e61aa54f3d00c4adb43ed4fd90 100644
--- a/pkg/pkg.status
+++ b/pkg/pkg.status
@@ -52,7 +52,6 @@ analyzer/test/generated/parser_test: Pass, Timeout
 code_transformers/test/resolver_test: Pass, Timeout
 docgen/test/*: Skip # Slow
 polymer/test/build/all_phases_test: Skip # Slow
-polymer_expressions/test/globals_test: Pass, Timeout
 smoke/test/codegen/end_to_end_test: Pass, Timeout
 smoke/test/codegen/recorder_test: Pass, Timeout
 template_binding/test/template_binding_test: Pass, Timeout
@@ -344,10 +343,6 @@ polymer_expressions/test/bindings_test: Skip
 third_party/html5lib/test/browser/browser_test: Skip
 http/test/html/*: Skip
 
-[ $runtime == safari || $runtime == safarimobilesim || $ie ]
-polymer_expressions/test/globals_test: Fail # Issue 16568
-polymer_expressions/test/bindings_test: Fail # Issue 16568
-
 [ $browser ]
 docgen/test/*: Skip  # Uses dart:io
 scheduled_test/test/scheduled_server_test: Skip # Uses dart:io