Fix an "unused" warning in Compiler

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=159473426

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

@@ -2110,7 +2110,8 @@ private List<CompilerInput> parsePotentialModules(List<CompilerInput> inputsToPr
     }
     for (CompilerInput input : filteredInputs) {
       input.setCompiler(this);
-      Node root = input.getAstRoot(this);
+      // Call getAstRoot to force parsing to happen.
+      input.getAstRoot(this);
       input.getRequires();
     }
     return filteredInputs;