[wasm] Patch trapping position into stack trace

src/js/messages.js

Index: src/js/messages.js
diff --git a/src/js/messages.js b/src/js/messages.js
index d99b920815d622ec84f4738ab95f32b809fecc49..4d683e76a384c44b579471c521bba701256dffdb 100644
--- a/src/js/messages.js
+++ b/src/js/messages.js
@@ -836,7 +836,10 @@ function GetStackFrames(raw_stack) {
     var fun = raw_stack[i + 1];
     var code = raw_stack[i + 2];
     var pc = raw_stack[i + 3];
-    var pos = %FunctionGetPositionForOffset(code, pc);
+    // For traps in wasm, the bytecode offset is passed as (-1 - offset).
+    // Otherwise, lookup the position from the pc.
+    var pos = IS_NUMBER(fun) && pc < 0 ? (-1 - pc) :
+      %FunctionGetPositionForOffset(code, pc);
     sloppy_frames--;
     frames.push(new CallSite(recv, fun, pos, (sloppy_frames < 0)));
   }