When removing asserts preserve the assert tightened type on the remai…

…ning expression.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125255697

src/com/google/javascript/jscomp/ClosureCodeRemoval.java

@@ -230,7 +230,9 @@ public void process(Node externs, Node root) {
         if (firstArg == null) {
           parent.replaceChild(call, NodeUtil.newUndefinedNode(call));
         } else {
-          parent.replaceChild(call, firstArg.detachFromParent());
+          Node replacement = firstArg.detachFromParent();
+          replacement.setJSType(call.getJSType());
+          parent.replaceChild(call, replacement);
         }
       }
       compiler.reportCodeChange();