Tightening up dart2js load timing.

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 21e7b5ca20fb720a8661c077e3daa58b0f9b5429..27c1add04b4797c83ae61f9e8c6366d52f70bfd2 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -857,11 +857,7 @@ class CodeEmitterTask extends CompilerTask {
     return js.fun('oldIsolate', [
       js('var isolateProperties = oldIsolate.${namer.isolatePropertiesName}'),
 
-      js(r'isolateProperties.$currentScript ='
-              'typeof document == "object" ?'
-              '(document.currentScript ||'
-                     'document.scripts[document.scripts.length - 1]) :'
-              'null'),
+      js(r'isolateProperties.$currentScript = null'),
 
       js('var isolatePrototype = oldIsolate.prototype'),
       js('var str = "{\\n"'),
@@ -2962,24 +2958,47 @@ class CodeEmitterTask extends CompilerTask {
       buffer.write(N);
     }
     addComment('BEGIN invoke [main].', buffer);
+    // This code finds the currently executing script by listening to the
+    // onload event of all script tags and getting the first script which
+    // finishes. Since onload is called immediately after execution this should
+    // not substantially change execution order.
     buffer.write("""
-if (typeof document !== "undefined" && document.readyState !== "complete") {
-  document.addEventListener("readystatechange", function () {
-    if (document.readyState == "complete") {
-      if (typeof dartMainRunner === "function") {
-        dartMainRunner(function() { ${mainCall}; });
-      } else {
-        ${mainCall};
-      }
+;(function (callback) {
+  if (typeof document === 'undefined') {
+    callback(null);
+    return;
+  }
+  if (document.currentScript) {
+    callback(document.currentScript);
+    return;
+  }
+
+  var scripts = document.scripts;
+  function onLoad() {
+    for (var i = 0; i < scripts.length; ++i) {
+      scripts[i].removeEventListener('load', onLoad, false);
     }
-  }, false);
-} else {
+    callback(event.target);
+  }
+  for (var i = 0; i < scripts.length; ++i) {
+    scripts[i].addEventListener('load', onLoad, false);
+  }
+})(function(currentScript) {
+  ${namer.isolateName}.${namer.isolatePropertiesName}.\$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 {
     ${mainCall};
   }
-}
+});
 """);
     addComment('END invoke [main].', buffer);
   }