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 27770a92f191954251a8ccf301773ab026f63282..9eb89177708548289daeeb50b15cd4f219f908ca 100644
--- a/runtime/lib/errors.cc
+++ b/runtime/lib/errors.cc
@@ -62,8 +62,9 @@ static RawScript* FindScript(DartFrameIterator* iterator) {
 // Allocate and throw a new AssertionError.
 // Arg0: index of the first token of the failed assertion.
 // Arg1: index of the first token after the failed assertion.
+// Arg2: Message object or null.
 // 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.
   const TokenPosition assertion_start =
@@ -71,7 +72,8 @@ DEFINE_NATIVE_ENTRY(AssertionError_throwNew, 2) {
   const TokenPosition assertion_end =
       TokenPosition(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.
@@ -92,6 +94,7 @@ 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();