Add some scope-related DCHECKs.

src/ast/variables.h

Index: src/ast/variables.h
diff --git a/src/ast/variables.h b/src/ast/variables.h
index e1ac44616e85e01de42025a18e754fabda9f373a..f1f63b8a14cb8f9bf151dc3e2c7f9db248995486 100644
--- a/src/ast/variables.h
+++ b/src/ast/variables.h
@@ -43,6 +43,7 @@ class Variable final : public ZoneObject {
     return force_context_allocation_;
   }
   void ForceContextAllocation() {
+    DCHECK(IsUnallocated() || IsContextSlot());

[Toon Verwaest, 2016/08/19 12:48:52]

This is surprising. I'd hope we wouldn't call ForceContextAllocation on already
allocated variables. I'm fine with cleaning this up in another CL, but I think
that should be an invariant ...

     force_context_allocation_ = true;
   }
   bool is_used() { return is_used_; }
@@ -96,6 +97,7 @@ class Variable final : public ZoneObject {
   }
 
   void AllocateTo(VariableLocation location, int index) {
+    DCHECK(IsUnallocated() || (location_ == location && index_ == index));

[Toon Verwaest, 2016/08/19 12:48:52]

Same here...

     location_ = location;
     index_ = index;
   }