Add thread safe constant canonicalization.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 3fc77801418f679d91e194c09b450adf08359dc2..70cca1ab368e7c03d20569c3f4be49ce83ecb902 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -791,6 +791,7 @@ Isolate::Isolate(const Dart_IsolateFlags& api_flags)
       mutex_(new Mutex()),
       symbols_mutex_(new Mutex()),
       type_canonicalization_mutex_(new Mutex()),
+      constant_canonicalization_mutex_(new Mutex()),
       saved_stack_limit_(0),
       deferred_interrupts_mask_(0),
       deferred_interrupts_(0),
@@ -856,6 +857,8 @@ Isolate::~Isolate() {
   symbols_mutex_ = NULL;
   delete type_canonicalization_mutex_;
   type_canonicalization_mutex_ = NULL;
+  delete constant_canonicalization_mutex_;
+  constant_canonicalization_mutex_ = NULL;
   delete message_handler_;
   message_handler_ = NULL;  // Fail fast if we send messages to a dead isolate.
   ASSERT(deopt_context_ == NULL);  // No deopt in progress when isolate deleted.