Remove invalid assertion in store buffer code.

runtime/vm/store_buffer.cc

Index: runtime/vm/store_buffer.cc
diff --git a/runtime/vm/store_buffer.cc b/runtime/vm/store_buffer.cc
index b257c0aaf15ada53997e70a4969626bdfe6fe9d1..67f0fcbbf90099cdf3193211b153966c41ec6cd0 100644
--- a/runtime/vm/store_buffer.cc
+++ b/runtime/vm/store_buffer.cc
@@ -104,7 +104,6 @@ void StoreBuffer::PushBlock(Block* block, ThresholdPolicy policy) {
     // Sanity check: it makes no sense to schedule the GC in another isolate.
     // (If Isolate ever gets multiple store buffers, we should avoid this
     // coupling by passing in an explicit callback+parameter at construction.)
-    ASSERT(thread->isolate()->mutator_thread() == thread);
     ASSERT(thread->isolate()->store_buffer() == this);
     thread->ScheduleInterrupts(Thread::kVMInterrupt);
   }