Fix error reporting for System.getProperty magic method nodes (#9812)

An earlier iteration appeared to add getProperty to GWT.java instead of
using System. In that implementation, the "answers" for rebind seemed
to be reused to hold default expressions, but that also changed before
it was submitted to GWT.

Fixes #9806

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

@@ -731,15 +731,13 @@ public boolean visit(JPermutationDependentValue x, Context ctx) {
       print(".class)");
     } else {
       assert x.isProperty();
-      print("GWT.getProperty(");
+      print("System.getProperty(");
       print("\"");
       print(x.getRequestedValue());
       print("\"");
-      if (x.getResultValues().get(0) != null) {
-        print(",");
-        print("\"");
-        print(x.getResultValues().get(0));
-        print("\"");
+      if (x.getDefaultValueExpression() != null) {
+        print(", ");
+        accept(x.getDefaultValueExpression());
       }
       print(")");
     }