prevent NPE while dumping AST

Bug: #9605
Bug-Link: https://github.com/gwtproject/gwt/issues/9605
Change-Id: I6bf1d45bcaf9e4ee9474db17187ea5faad5438ca

dev/core/src/com/google/gwt/dev/jjs/impl/ToStringGenerationVisitor.java

diff --git a/dev/core/src/com/google/gwt/dev/jjs/impl/ToStringGenerationVisitor.java b/dev/core/src/com/google/gwt/dev/jjs/impl/ToStringGenerationVisitor.java
index ece34c1..c9a3077 100644
--- a/dev/core/src/com/google/gwt/dev/jjs/impl/ToStringGenerationVisitor.java
+++ b/dev/core/src/com/google/gwt/dev/jjs/impl/ToStringGenerationVisitor.java
@@ -604,7 +604,7 @@
       return false;
     }
     printMethodHeader(x);
-    if (x.isAbstract()) {
+    if (x.isAbstract() || x.isJsNative()) {
       semi();
       newlineOpt();
     } else {