fix spacing in error message

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168050011

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

@@ -4584,7 +4584,7 @@ static boolean evaluatesToLocalValue(Node value, Predicate<Node> locals) {
         }
 
         throw new IllegalStateException(
-            "Unexpected expression node" + value + "\n parent:" + value.getParent());
+            "Unexpected expression node: " + value + "\n parent:" + value.getParent());
     }
   }