Set supported Java source level to "9" (#182)

Fixes #181

Author: sormuras

core/src/main/java/com/google/googlejavaformat/java/RemoveUnusedImports.java

@@ -53,6 +53,7 @@
 import org.openjdk.source.util.TreeScanner;
 import org.openjdk.tools.javac.api.JavacTrees;
 import org.openjdk.tools.javac.file.JavacFileManager;
+import org.openjdk.tools.javac.main.Option;
 import org.openjdk.tools.javac.parser.JavacParser;
 import org.openjdk.tools.javac.parser.ParserFactory;
 import org.openjdk.tools.javac.tree.DCTree;
@@ -195,6 +196,8 @@ public Void visitIdentifier(IdentifierTree node, Void aVoid) {
   public static String removeUnusedImports(
       final String contents, JavadocOnlyImports javadocOnlyImports) {
     Context context = new Context();
+    // TODO(cushon): this should default to the latest supported source level, same as in Formatter
+    Options.instance(context).put(Option.SOURCE, "9");
     JCCompilationUnit unit = parse(context, contents);
     if (unit == null) {
       // error handling is done during formatting

core/src/test/java/com/google/googlejavaformat/java/RemoveUnusedImportsTest.java

@@ -329,6 +329,18 @@ public static Collection<Object[]> parameters() {
           "public class Test implements Foo, Bar, Baz, Bork {}",
         },
       },
+      {
+        {
+          "import java.lang.Foo;", //
+          "interface Test { private static void foo() {} }",
+        },
+        {
+          "interface Test { private static void foo() {} }",
+        },
+        {
+          "interface Test { private static void foo() {} }",
+        },
+      },
     };
     ImmutableList.Builder<Object[]> builder = ImmutableList.builder();
     for (String[][] inputAndOutput : inputsOutputs) {