Make Java 8 the default -sourceLevel.

Change-Id: I596bab8b80b2ccf173ff0658f2e91c2d444deab1
diff --git a/dev/core/src/com/google/gwt/dev/util/arg/ArgHandlerSourceLevel.java b/dev/core/src/com/google/gwt/dev/util/arg/ArgHandlerSourceLevel.java
index 993d6dc..17713ac 100644
--- a/dev/core/src/com/google/gwt/dev/util/arg/ArgHandlerSourceLevel.java
+++ b/dev/core/src/com/google/gwt/dev/util/arg/ArgHandlerSourceLevel.java
@@ -32,12 +32,12 @@
 
   @Override
   public String[] getDefaultArgs() {
-    return new String[] {getTag(), AUTO_SELECT};
+    return new String[] {getTag(), SourceLevel.JAVA8.getStringValue()};
   }
 
   @Override
   public String getPurpose() {
-    return "Specifies Java source level (defaults to " + AUTO_SELECT + ":"
+    return "Specifies Java source level (defaults to " + SourceLevel.JAVA8.getStringValue() + ":"
         + SourceLevel.DEFAULT_SOURCE_LEVEL + ")";
   }