[modules] Fix bugs in assignments to exported variables.

- Add hole check if needed.
- Preserve the accumulator so that the result is the rhs.

R=adamk@chromium.org
BUG=v8:1569,v8:5547

Review-Url: https://chromiumcodereview.appspot.com/2438653003
Cr-Commit-Position: refs/heads/master@{#40510}

src/interpreter/bytecode-generator.cc

@@ -2042,8 +2042,14 @@ void BytecodeGenerator::BuildVariableAssignment(Variable* variable,
       builder()
           ->StoreAccumulatorInRegister(args[1])
           .LoadLiteral(it->second->export_name->string())
-          .StoreAccumulatorInRegister(args[0])
-          .CallRuntime(Runtime::kStoreModuleExport, args);
+          .StoreAccumulatorInRegister(args[0]);
+      if (needs_hole_check) {
+        builder()->CallRuntime(Runtime::kLoadModuleExport, args[0]);
+        BuildHoleCheckForVariableAssignment(variable, op);
+      }
+      builder()
+          ->CallRuntime(Runtime::kStoreModuleExport, args)
+          .LoadAccumulatorWithRegister(args[1]);
       break;
     }
   }

test/mjsunit/modules-init3.js

@@ -11,6 +11,11 @@ assertThrows(() => x, ReferenceError);
 assertThrows(() => y, ReferenceError);
 assertThrows(() => z, ReferenceError);
 
+assertEquals(23, w = 23);
+assertThrows(() => x = 666, ReferenceError);
+assertThrows(() => y = 666, ReferenceError);
+assertThrows(() => z = 666, TypeError);
+
 export function* v() { return 40 }
 export var w = 41;
 export let x = 42;

test/mjsunit/modules-skip-init3.js

@@ -10,6 +10,12 @@ assertThrows(() => x, ReferenceError);
 assertThrows(() => y, ReferenceError);
 assertThrows(() => z, ReferenceError);
 
+assertThrows(() => v = 666, TypeError);
+assertThrows(() => w = 666, TypeError);
+assertThrows(() => x = 666, TypeError);
+assertThrows(() => y = 666, TypeError);
+assertThrows(() => z = 666, TypeError);
+
 export function check() {
   assertEquals({value: 40, done: true}, v().next());
   assertEquals(41, w);