Bug 1207882 - Associate an install target with every ContextDerived object. r=gps

Since reading context['FINAL_TARGET'] can have side effects, use it lazily.

python/mozbuild/mozbuild/frontend/data.py

@@ -88,6 +88,12 @@ class ContextDerived(TreeMetadata):
 
         self.config = context.config
 
+        self._context = context
+
+    @property
+    def install_target(self):
+        return self._context['FINAL_TARGET']
+
     @property
     def relobjdir(self):
         return mozpath.relpath(self.objdir, self.topobjdir)
@@ -163,7 +169,6 @@ class XPIDLFile(ContextDerived):
     __slots__ = (
         'add_to_manifest',
         'basename',
-        'install_target',
         'source_path',
     )
 
@@ -175,8 +180,6 @@ class XPIDLFile(ContextDerived):
         self.module = module
         self.add_to_manifest = add_to_manifest
 
-        self.install_target = context['FINAL_TARGET']
-
 class BaseDefines(ContextDerived):
     """Context derived container object for DEFINES/HOST_DEFINES,
     which are OrderedDicts.