Report NullPointerException when we try to construct a regular expression

vm/ast.h

 // Copyright (c) 2011, 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.
 
 #ifndef VM_AST_H_
 #define VM_AST_H_
 
 #include "vm/allocation.h"
 #include "vm/assert.h"
 #include "vm/growable_array.h"
@@ skipping 297 matching lines @@
   LiteralNode(intptr_t token_index, const Instance& literal)
       : AstNode(token_index), literal_(literal) {
     ASSERT(literal.IsZoneHandle());
 #if defined(DEBUG)
     if (literal.IsString()) {
       String& str = String::Handle();
       str ^= literal.raw();
       ASSERT(str.IsSymbol());
     }
 #endif  // defined(DEBUG)
-    ASSERT(literal.IsNull() || Class::Handle(literal.clazz()).is_finalized());
+    ASSERT(literal.IsNull() ||
+           Class::Handle(literal.clazz()).is_finalized() ||
+           Class::Handle(literal.clazz()).is_prefinalized());
   }
 
   const Instance& literal() const { return literal_; }
 
   virtual const Instance* EvalConstExpr() const {
     return &literal();
   }
 
   virtual void VisitChildren(AstNodeVisitor* visitor) const { }
 
@@ skipping 1546 matching lines @@
   const LocalVariable& context_var_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(InlinedFinallyNode);
 };
 
 }  // namespace dart
 
 #undef DECLARE_COMMON_NODE_FUNCTIONS
 
 #endif  // VM_AST_H_