Bug 1335886, part 2 - Nodes never have existing symbol tables in enterScope. r=billm

This code gets deleted in the next patch, but the point here is that
we can still compile Firefox with this assertion in place.

MozReview-Commit-ID: 33dw0xm7L4R

--HG--
extra : rebase_source : 140fca3b0b5abf5b3d6636ea63bae2adf1311b09

ipc/ipdl/ipdl/type.py

@@ -445,8 +445,8 @@ class SymbolTable:
     def enterScope(self, node):
         assert isinstance(self.scopes[0], dict)
         assert isinstance(self.currentScope, dict)
+        assert not hasattr(node, 'symtab')
 
-        if not hasattr(node, 'symtab'):
         node.symtab = { }
 
         self.scopes.append(node.symtab)