Improve fix to prepare for a change to the return type of `JCCompilat…

…ionUnit#getImports` in an upcoming JDK version

Follow-up to 9e0fbf7

This allows the code to work if it's compiled and executed against different javac versions.

PiperOrigin-RevId: 655688755

core/src/main/java/com/google/errorprone/refaster/ImportPolicy.java

@@ -266,7 +266,7 @@ private static ImmutableSet<String> getAllImports(Inliner inliner, WhichImports
             whichImports.getExistingImports(inliner),
             Optional.ofNullable(inliner.getContext())
                 .map(c -> c.get(JCCompilationUnit.class))
-                .map(JCCompilationUnit::getImports)
+                .map(ImportPolicy::getImports)
                 .map(Collection::stream)
                 .orElse(Stream.of())
                 .filter(JCImport.class::isInstance)
@@ -276,6 +276,15 @@ private static ImmutableSet<String> getAllImports(Inliner inliner, WhichImports
         .collect(toImmutableSet());
   }
 
+  @SuppressWarnings("unchecked")
+  private static Collection<JCTree> getImports(JCCompilationUnit unit) {
+    try {
+      return (Collection<JCTree>) JCCompilationUnit.class.getMethod("getImports").invoke(unit);
+    } catch (ReflectiveOperationException e) {
+      throw new LinkageError(e.getMessage(), e);
+    }
+  }
+
   private static JCTree getQualifiedIdentifier(JCImport i) {
     try {
       return (JCTree) JCImport.class.getMethod("getQualifiedIdentifier").invoke(i);