Issue 1043: Missing --documentation-out should default to "out" instead of asserting

compiler/java/com/google/dart/compiler/CommandLineOptions.java

Index: compiler/java/com/google/dart/compiler/CommandLineOptions.java
diff --git a/compiler/java/com/google/dart/compiler/CommandLineOptions.java b/compiler/java/com/google/dart/compiler/CommandLineOptions.java
index ca34a460a4cdca41eeb15c1946f4191f767839b7..fab93082999dbdbd19413e0201988e65dfaab7a4 100644
--- a/compiler/java/com/google/dart/compiler/CommandLineOptions.java
+++ b/compiler/java/com/google/dart/compiler/CommandLineOptions.java
@@ -60,7 +60,7 @@ public class CommandLineOptions {
 
     @Option(name = "--documentation-out", aliases = { "-documentation-out" },
         usage = "Directory to write documentation")
-    private String documentationOutputDirectory = null;
+    private String documentationOutputDirectory = "out";
 
     @Option(name = "--generate-documentation", aliases = { "-generate-documentation" },
         usage = "Generate docs from source")