Make type canonicalization thread safe

runtime/vm/object.h

Index: runtime/vm/object.h
diff --git a/runtime/vm/object.h b/runtime/vm/object.h
index 7c20cae3d325d3e57d4bb0243fcd842d0a51f9ab..8fafabc0eff3d3ac9578b46d4377e64f24ea382b 100644
--- a/runtime/vm/object.h
+++ b/runtime/vm/object.h
@@ -1488,6 +1488,12 @@ class Class : public Object {
       TrailPtr bound_trail,
       Heap::Space space);
 
+  // Returns AbstractType::null() if type not found.
+  RawAbstractType* LookupCanonicalType(Zone* zone,
+                                       const AbstractType& type,
+                                       intptr_t* index) const;
+
+  // Returns canonical type. Thread safe.
   RawAbstractType* LookupOrAddCanonicalType(const AbstractType& type,
                                             intptr_t start_index) const;