Show callstack in browser upon REPL crashes

scripts/test-runner.js

@@ -286,6 +286,7 @@ function runTest(name, code, options: PrepackOptions, args) {
   options.invariantLevel = code.includes("// omit invariants") || args.verbose ? 0 : 99;
   if (code.includes("// emit concrete model")) options.emitConcreteModel = true;
   if (code.includes("// exceeds stack limit")) options.maxStackDepth = 10;
+  if (code.includes("// heapGraph")) options.heapGraphFormat = "VISJS";
   if (code.includes("// react")) {
     options.reactEnabled = true;
     options.reactOutput = "jsx";

src/utils/generator.js

@@ -253,9 +253,15 @@ class ModifiedBindingEntry extends GeneratorEntry {
       containingGenerator === this.containingGenerator,
       "This entry requires effects to be applied and may not be moved"
     );
-    invariant(this.modifiedBinding.value === this.newValue);
+    invariant(
+      this.modifiedBinding.value === this.newValue,
+      "ModifiedBinding's value has been changed since last visit."
+    );
     let [residualBinding, newValue] = context.visitModifiedBinding(this.modifiedBinding);
-    invariant(this.residualFunctionBinding === undefined || this.residualFunctionBinding === residualBinding);
+    invariant(
+      this.residualFunctionBinding === undefined || this.residualFunctionBinding === residualBinding,
+      "ResidualFunctionBinding has been changed since last visit."
+    );
     this.residualFunctionBinding = residualBinding;
     this.newValue = newValue;
     return true;

website/js/repl-worker.js

@@ -42,7 +42,7 @@ onmessage = function(e) {
     }
   } catch (err) {
     buffer.push({
-      message: err.message || 'An unknown error occurred'
+      message: err.stack || 'An unknown error occurred'
     });
     postMessage({ type: 'error', data: buffer });
   }