CF backend: Omit writing code for native methods

Native methods have no code object and should not have any code in the
classfile. Avoid NullPointerException in that case.

Change-Id: Ib3ed0f13672452d668fad9a8ebf7b664fef836c2
diff --git a/src/main/java/com/android/tools/r8/jar/CfApplicationWriter.java b/src/main/java/com/android/tools/r8/jar/CfApplicationWriter.java
index f9c73ac..fd823f0 100644
--- a/src/main/java/com/android/tools/r8/jar/CfApplicationWriter.java
+++ b/src/main/java/com/android/tools/r8/jar/CfApplicationWriter.java
@@ -145,7 +145,7 @@
     String signature = null; // TODO(zerny): Support generic signatures.
     String[] exceptions = null;
     MethodVisitor visitor = writer.visitMethod(access, name, desc, signature, exceptions);
-    if (!method.accessFlags.isAbstract()) {
+    if (!method.accessFlags.isAbstract() && !method.accessFlags.isNative()) {
       writeCode(method.getCode(), visitor);
     }
   }