Add optional message to assert in Dart2js.

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index 6692d2723c426a4e4f984899aac1ac55fb05e77b..65f603d5b5aeb25abf39e32478faf7f44211999d 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -3301,7 +3301,7 @@ class FallThroughErrorImplementation extends FallThroughError {
 /**
  * Helper function for implementing asserts. The compiler treats this specially.
  */
-void assertHelper(condition) {
+bool assertConditionHelper(condition) {
   // Do a bool check first because it is common and faster than 'is Function'.
   if (condition is !bool) {
     if (condition is Function) condition = condition();
@@ -3311,7 +3311,14 @@ void assertHelper(condition) {
   }
   // Compare to true to avoid boolean conversion check in checked
   // mode.
-  if (true != condition) throw new AssertionError();
+  return (true == condition);
+}
+
+/**
+ * Helper function for implementing asserts. The compiler treats this specially.
+ */
+void assertThrowHelper(Object message) {
+  throw new AssertionError(message);
 }
 
 /**