Record a correct deoptimization environment for slow-path code.

runtime/vm/intermediate_language.h

Index: runtime/vm/intermediate_language.h
diff --git a/runtime/vm/intermediate_language.h b/runtime/vm/intermediate_language.h
index e7cf25312c8310b6addc165e7e7bbe82d5fe1419..baf8ac98100be2ed700930eee930840453105ae1 100644
--- a/runtime/vm/intermediate_language.h
+++ b/runtime/vm/intermediate_language.h
@@ -6753,6 +6753,8 @@ class Environment : public ZoneAllocated {
 
   const Function& function() const { return function_; }
 
+  Environment* DeepCopy() const { return DeepCopy(Length()); }
+
   void DeepCopyTo(Instruction* instr) const;
   void DeepCopyToOuter(Instruction* instr) const;
 
@@ -6773,10 +6775,9 @@ class Environment : public ZoneAllocated {
         function_(function),
         outer_(outer) { }
 
-  // Deep copy an environment.  A 'length' parameter can be given, which may
-  // be less than the environment's length in order to drop values (e.g.,
-  // passed arguments) from the copy.
-  Environment* DeepCopy() const { return DeepCopy(Length()); }
+  // Deep copy an environment.  The 'length' parameter may be less than the
+  // environment's length in order to drop values (e.g., passed arguments)
+  // from the copy.
   Environment* DeepCopy(intptr_t length) const;
 
   GrowableArray<Value*> values_;