Prevent mutation of final variables

runtime/vm/ast.cc

Index: runtime/vm/ast.cc
===================================================================
--- runtime/vm/ast.cc	(revision 14350)
+++ runtime/vm/ast.cc	(working copy)
@@ -315,6 +315,9 @@
 
 
 AstNode* LoadStaticFieldNode::MakeAssignmentNode(AstNode* rhs) {
+  if (field().is_final()) {
+    return NULL;
+  }
   return new StoreStaticFieldNode(token_pos(), field(), rhs);
 }
 
@@ -362,6 +365,9 @@
     const Field& field =
         Field::ZoneHandle(cls().LookupStaticField(field_name()));
     if (!field.IsNull()) {
+      if (field.is_final()) {
+        return NULL;
+      }
 #if defined(DEBUG)
       const String& getter_name =
           String::Handle(Field::GetterName(field_name()));