Add optional message to assert in Dart2js.

pkg/compiler/lib/src/dart_backend/placeholder_collector.dart

Index: pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
diff --git a/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart b/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
index ed0c6bc9ee3ba4d8a396671d6987838b53536b0e..b15e76a81dc0480c331c846a0f8a35a9e395090c 100644
--- a/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
+++ b/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
@@ -45,9 +45,7 @@ class SendVisitor extends Visitor {
 
   visitSend(Send node) {
     Element element = elements[node];
-    if (elements.isAssert(node)) {
-      return;
-    } else if (elements.isTypeLiteral(node)) {
+    if (elements.isTypeLiteral(node)) {
       DartType type = elements.getTypeLiteralType(node);
       if (!type.isDynamic) {
         if (type is TypeVariableType) {
@@ -139,7 +137,6 @@ class SendVisitor extends Visitor {
         collector.currentElement, element, node);
 
     if (Elements.isUnresolved(element)
-        || elements.isAssert(node)
         || element.isDeferredLoaderGetter) {
       return;
     }