Fix for NPE with StringLiteral in toString()

compiler/java/com/google/dart/compiler/ast/DartToSourceVisitor.java

Index: compiler/java/com/google/dart/compiler/ast/DartToSourceVisitor.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartToSourceVisitor.java b/compiler/java/com/google/dart/compiler/ast/DartToSourceVisitor.java
index 6bedf8ccf5d8ceaf3400a87ff878f81059d2fff2..0ad9d5858b34cc2402e094c50cc8e4ffbfb62a1e 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartToSourceVisitor.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartToSourceVisitor.java
@@ -816,6 +816,9 @@ public class DartToSourceVisitor extends ASTVisitor<Void> {
 
   @Override
   public Void visitStringLiteral(DartStringLiteral x) {
+    if (x.getValue() == null) {
+      return null;
+    }
     p("\"");
     // 'replaceAll' takes regular expressions as first argument and parses the second argument
     // for captured groups. We must escape backslashes twice: once to escape them in the source