Removing dart2js main execution warning message

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index 3118874217d4d8cebfdf1478105c9cb3c626cab5..4cd167e789e93e0e41b14ca22b82faf81cbbd011 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -3016,12 +3016,6 @@ class CodeEmitterTask extends CompilerTask {
 })(function(currentScript) {
   init.currentScript = currentScript;
 
-  if (typeof console !== "undefined" && typeof document !== "undefined" &&
-      document.readyState == "loading") {
-    console.warn("Dart script executed synchronously, use <script src='" +
-        currentScript.src + "' defer></scr" + "ipt> to execute after parsing " +
-        "has completed. See also http://dartbug.com/12281.");
-  }
   if (typeof dartMainRunner === "function") {
     dartMainRunner(function() { ${mainCall}; });
   } else {