LFInfo missing with JavaScript backend

The JS backend produces the following warnings because it doesn't compute the LambdaFormInfo required to write the interface file:

+WARNING:
+  updateDecl
+  Name without LFInfo: foo
+  Call stack:
+      CallStack (from HasCallStack):
+        warnPprTrace, called at compiler/GHC/Iface/Make.hs:<line>:<column> in <package-id>:GHC.Iface.Make

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