Make type canonicalization thread safe

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 614ab37f4b8582ff528e080019e69490951b689b..b72a80d51ac57bb6d0468b144b4e0753fe73e1f6 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -790,6 +790,7 @@ Isolate::Isolate(const Dart_IsolateFlags& api_flags)
       simulator_(NULL),
       mutex_(new Mutex()),
       symbols_mutex_(new Mutex()),
+      type_canonicalization_mutex_(new Mutex()),
       saved_stack_limit_(0),
       deferred_interrupts_mask_(0),
       deferred_interrupts_(0),
@@ -853,6 +854,8 @@ Isolate::~Isolate() {
   mutex_ = NULL;  // Fail fast if interrupts are scheduled on a dead isolate.
   delete symbols_mutex_;
   symbols_mutex_ = NULL;
+  delete type_canonicalization_mutex_;
+  type_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.