Don't crash attempting to JSON print an uninitialized Map.

runtime/vm/object.h

 // 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.
 
 #ifndef RUNTIME_VM_OBJECT_H_
 #define RUNTIME_VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 8182 matching lines @@
 
   RawSmi* deleted_keys() const { return raw_ptr()->deleted_keys_; }
   void SetDeletedKeys(intptr_t value) const {
     StoreSmi(&raw_ptr()->deleted_keys_, Smi::New(value));
   }
   static intptr_t deleted_keys_offset() {
     return OFFSET_OF(RawLinkedHashMap, deleted_keys_);
   }
 
   intptr_t Length() const {
+    // The map may be uninitialized.
+    if (raw_ptr()->used_data_ == Object::null()) return 0;
+    if (raw_ptr()->deleted_keys_ == Object::null()) return 0;
+
     intptr_t used = Smi::Value(raw_ptr()->used_data_);
     intptr_t deleted = Smi::Value(raw_ptr()->deleted_keys_);
     return (used >> 1) - deleted;
   }
 
   // This iterator differs somewhat from its Dart counterpart (_CompactIterator
   // in runtime/lib/compact_hash.dart):
   //  - There are no checks for concurrent modifications.
   //  - Accessing a key or value before the first call to MoveNext and after
   //    MoveNext returns false will result in crashes.
@@ skipping 619 matching lines @@
 
 inline void TypeArguments::SetHash(intptr_t value) const {
   // This is only safe because we create a new Smi, which does not cause
   // heap allocation.
   StoreSmi(&raw_ptr()->hash_, Smi::New(value));
 }
 
 }  // namespace dart
 
 #endif  // RUNTIME_VM_OBJECT_H_