dart2js: fix a few TODOs in the startup emitter.

pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
index 42d0e005c91a4693a389ae29d475bdfea09e9034..60382c168c88685200eede50b1df1a4b7a497afb 100644
--- a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
@@ -848,7 +848,14 @@ class Emitter implements js_emitter.Emitter {
           NativeGenerator.generateIsolateAffinityTagInitialization(
               backend,
               generateEmbeddedGlobalAccess,
-              js("convertToFastObject", [])));
+              js("""
+        // On V8, the 'intern' function converts a string to a symbol, which
+        // makes property access much faster.
+        function (s) {
+          var o = {};
+          o[s] = 1;
+          return Object.keys(convertToFastObject(o))[0];
+        }""", [])));
     }
 
     parts..add(js.comment('BEGIN invoke [main].'))