Background compilation work:

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 VM_OBJECT_H_
 #define VM_OBJECT_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "platform/utils.h"
@@ skipping 4375 matching lines @@
   bool IsAllocationStubCode() const;
   bool IsStubCode() const;
   bool IsFunctionCode() const;
 
   void DisableDartCode() const;
 
   void DisableStubCode() const;
 
   void Enable() const {
     if (!IsDisabled()) return;
+    ASSERT(Thread::Current()->IsMutatorThread());
     ASSERT(instructions() != active_instructions());
     set_active_instructions(instructions());
   }
 
   bool IsDisabled() const {
     return instructions() != active_instructions();
   }
 
  private:
   void set_state_bits(intptr_t bits) const;
@@ skipping 34 matching lines @@
     return Code::OptimizedBit::decode(code->ptr()->state_bits_);
   }
 
   static const intptr_t kEntrySize = sizeof(int32_t);  // NOLINT
 
   void set_compile_timestamp(int64_t timestamp) const {
     StoreNonPointer(&raw_ptr()->compile_timestamp_, timestamp);
   }
 
   void set_active_instructions(RawInstructions* instructions) const {
+    ASSERT(Thread::Current()->IsMutatorThread() || !is_alive());
     // RawInstructions are never allocated in New space and hence a
     // store buffer update is not needed here.
     StorePointer(&raw_ptr()->active_instructions_, instructions);
     StoreNonPointer(&raw_ptr()->entry_point_,
                     reinterpret_cast<uword>(instructions->ptr()) +
                     Instructions::HeaderSize());
   }
 
   void set_instructions(RawInstructions* instructions) const {
+    ASSERT(Thread::Current()->IsMutatorThread() || !is_alive());
     StorePointer(&raw_ptr()->instructions_, instructions);
   }
 
   void set_pointer_offsets_length(intptr_t value) {
     // The number of fixups is limited to 1-billion.
     ASSERT(Utils::IsUint(30, value));
     set_state_bits(PtrOffBits::update(value, raw_ptr()->state_bits_));
   }
   int32_t* PointerOffsetAddrAt(int index) const {
     ASSERT(index >= 0);
@@ skipping 3738 matching lines @@
 
 
 RawObject* MegamorphicCache::GetTargetFunction(const Array& array,
                                                intptr_t index) {
   return array.At((index * kEntryLength) + kTargetFunctionIndex);
 }
 
 }  // namespace dart
 
 #endif  // VM_OBJECT_H_