Fix some more shorten-64-to-32 warnings:

runtime/vm/thread.h

 // Copyright (c) 2015, 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_THREAD_H_
 #define VM_THREAD_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 559 matching lines @@
   REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_FIELDS)
 #undef REUSABLE_HANDLE_FIELDS
 
 #if defined(DEBUG)
 #define REUSABLE_HANDLE_SCOPE_VARIABLE(object)                                 \
   bool reusable_##object##_handle_scope_active_;
   REUSABLE_HANDLE_LIST(REUSABLE_HANDLE_SCOPE_VARIABLE);
 #undef REUSABLE_HANDLE_SCOPE_VARIABLE
 #endif  // defined(DEBUG)
 
-  class AtSafepointField : public BitField<bool, 0, 1> {};
-  class SafepointRequestedField : public BitField<bool, 1, 1> {};
-  class BlockedForSafepointField : public BitField<bool, 2, 1> {};
+  class AtSafepointField : public BitField<uint32_t, bool, 0, 1> {};
+  class SafepointRequestedField : public BitField<uint32_t, bool, 1, 1> {};
+  class BlockedForSafepointField : public BitField<uint32_t, bool, 2, 1> {};
   uint32_t safepoint_state_;
   uint32_t execution_state_;
 
   Thread* next_;  // Used to chain the thread structures in an isolate.
 
   explicit Thread(Isolate* isolate);
 
   void StoreBufferRelease(
       StoreBuffer::ThresholdPolicy policy = StoreBuffer::kCheckThreshold);
   void StoreBufferAcquire();
@@ skipping 41 matching lines @@
 // Disable thread interrupts.
 class DisableThreadInterruptsScope : public StackResource {
  public:
   explicit DisableThreadInterruptsScope(Thread* thread);
   ~DisableThreadInterruptsScope();
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_H_