Add optional message argument to assert statements in the VM.

runtime/lib/errors.cc

Index: runtime/lib/errors.cc
diff --git a/runtime/lib/errors.cc b/runtime/lib/errors.cc
index 8383a307634ed1be18f7ce59591253d79ff75652..a0c4295dc68e98fa603cf14eb42933a71756b80a 100644
--- a/runtime/lib/errors.cc
+++ b/runtime/lib/errors.cc
@@ -14,15 +14,15 @@ namespace dart {
 // Arg0: index of the first token of the failed assertion.
 // Arg1: index of the first token after the failed assertion.
 // Return value: none, throws an exception.
-DEFINE_NATIVE_ENTRY(AssertionError_throwNew, 2) {
+DEFINE_NATIVE_ENTRY(AssertionError_throwNew, 3) {
   // No need to type check the arguments. This function can only be called
   // internally from the VM.
   intptr_t assertion_start =
       Smi::CheckedHandle(arguments->NativeArgAt(0)).Value();
   intptr_t assertion_end =
       Smi::CheckedHandle(arguments->NativeArgAt(1)).Value();
-
-  const Array& args = Array::Handle(Array::New(4));
+  const Instance& message = Instance::CheckedHandle(arguments->NativeArgAt(2));
+  const Array& args = Array::Handle(Array::New(5));
 
   DartFrameIterator iterator;
   iterator.NextFrame();  // Skip native call.
@@ -43,6 +43,7 @@ DEFINE_NATIVE_ENTRY(AssertionError_throwNew, 2) {
   args.SetAt(1, String::Handle(script.url()));
   args.SetAt(2, Smi::Handle(Smi::New(from_line)));
   args.SetAt(3, Smi::Handle(Smi::New(script.HasSource() ? from_column : -1)));
+  args.SetAt(4, message);
 
   Exceptions::ThrowByType(Exceptions::kAssertion, args);
   UNREACHABLE();