Fix VM crash when encountering an assignment expression in a case expression.

runtime/vm/parser.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/parser.h"
 #include "vm/flags.h"
 
 #ifndef DART_PRECOMPILED_RUNTIME
 
 #include "lib/invocation_mirror.h"
@@ skipping 11454 matching lines @@
       expr = LiteralIfStaticConst(Z, expr);
     }
     return expr;
   }
   // Assignment expressions.
   if (!IsLegalAssignableSyntax(expr, TokenPos())) {
     ReportError(expr_pos, "expression is not assignable");
   }
   const Token::Kind assignment_op = CurrentToken();
   const TokenPosition assignment_pos = TokenPos();
-  ConsumeToken();
-  const TokenPosition right_expr_pos = TokenPos();
-  if (require_compiletime_const && (assignment_op != Token::kASSIGN)) {
-    ReportError(right_expr_pos,
+  if (require_compiletime_const) {
+    ReportError(assignment_pos,
                 "expression is not a valid compile-time constant");
   }
+  ConsumeToken();
   AstNode* right_expr = ParseExpr(require_compiletime_const, consume_cascades);
   if (assignment_op != Token::kASSIGN) {
     // Compound assignment: store inputs with side effects into temp. locals.
     LetNode* let_expr = PrepareCompoundAssignmentNodes(&expr);
     AstNode* assigned_value =
         ExpandAssignableOp(assignment_pos, assignment_op, expr, right_expr);
     AstNode* assign_expr =
         CreateAssignmentNode(expr, assigned_value, expr_ident, expr_pos, true);
     ASSERT(assign_expr != NULL);
     let_expr->AddNode(assign_expr);
@@ skipping 3672 matching lines @@
     const ArgumentListNode& function_args,
     const LocalVariable* temp_for_last_arg,
     bool is_super_invocation) {
   UNREACHABLE();
   return NULL;
 }
 
 }  // namespace dart
 
 #endif  // DART_PRECOMPILED_RUNTIME