[parser,ast] Simplify MarkExpressionAsAssigned.

There's no need to return anything.

Also add a DCHECK to make sure that we never mark a variable proxy as assigned
that is already resolved (to avoid potential inconsistency with the variable's
maybe_assigned status).

R=littledan@chromium.org
CC=mstarzinger@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2504613002
Cr-Commit-Position: refs/heads/master@{#41072}

src/ast/ast.h

@@ -1662,6 +1662,7 @@ class VariableProxy final : public Expression {
 
   bool is_assigned() const { return IsAssignedField::decode(bit_field_); }
   void set_is_assigned() {
+    DCHECK(!is_resolved());
     bit_field_ = IsAssignedField::update(bit_field_, true);
   }
 

src/parsing/parser-base.h

@@ -2732,7 +2732,7 @@ ParserBase<Impl>::ParseAssignmentExpression(bool accept_IN, bool* ok) {
         MessageTemplate::kInvalidLhsInAssignment, CHECK_OK);
   }
 
-  expression = impl()->MarkExpressionAsAssigned(expression);
+  impl()->MarkExpressionAsAssigned(expression);
 
   Token::Value op = Next();  // Get assignment operator.
   if (op != Token::ASSIGN) {
@@ -2973,7 +2973,7 @@ typename ParserBase<Impl>::ExpressionT ParserBase<Impl>::ParseUnaryExpression(
     expression = CheckAndRewriteReferenceExpression(
         expression, beg_pos, scanner()->location().end_pos,
         MessageTemplate::kInvalidLhsInPrefixOp, CHECK_OK);
-    expression = impl()->MarkExpressionAsAssigned(expression);
+    impl()->MarkExpressionAsAssigned(expression);
     impl()->RewriteNonPattern(CHECK_OK);
 
     return factory()->NewCountOperation(op,
@@ -3013,7 +3013,7 @@ typename ParserBase<Impl>::ExpressionT ParserBase<Impl>::ParsePostfixExpression(
     expression = CheckAndRewriteReferenceExpression(
         expression, lhs_beg_pos, scanner()->location().end_pos,
         MessageTemplate::kInvalidLhsInPostfixOp, CHECK_OK);
-    expression = impl()->MarkExpressionAsAssigned(expression);
+    impl()->MarkExpressionAsAssigned(expression);
     impl()->RewriteNonPattern(CHECK_OK);
 
     Token::Value next = Next();

src/parsing/parser.h

@@ -824,12 +824,11 @@ class V8_EXPORT_PRIVATE Parser : public NON_EXPORTED_BASE(ParserBase<Parser>) {
 
   // Determine if the expression is a variable proxy and mark it as being used
   // in an assignment or with a increment/decrement operator.
-  V8_INLINE static Expression* MarkExpressionAsAssigned(
-      Expression* expression) {
-    VariableProxy* proxy =
-        expression != NULL ? expression->AsVariableProxy() : NULL;
-    if (proxy != NULL) proxy->set_is_assigned();
-    return expression;
+  V8_INLINE static void MarkExpressionAsAssigned(Expression* expression) {
+    DCHECK_NOT_NULL(expression);
+    if (expression->IsVariableProxy()) {
+      expression->AsVariableProxy()->set_is_assigned();
+    }
   }
 
   // Returns true if we have a binary expression between two numeric

src/parsing/preparser.h

@@ -1205,11 +1205,10 @@ class PreParser : public ParserBase<PreParser> {
   V8_INLINE static void CheckAssigningFunctionLiteralToProperty(
       PreParserExpression left, PreParserExpression right) {}
 
-  V8_INLINE static PreParserExpression MarkExpressionAsAssigned(
+  V8_INLINE static void MarkExpressionAsAssigned(
       PreParserExpression expression) {
     // TODO(marja): To be able to produce the same errors, the preparser needs
     // to start tracking which expressions are variables and which are assigned.
-    return expression;
   }
 
   V8_INLINE bool ShortcutNumericLiteralBinaryExpression(PreParserExpression* x,