Make all possible Type methods const

While here, mark inherited virtual methods as override methods.

Bug: 24303749
Test: Compiles, unittests

Change-Id: Ia7114b53e243b8539a6c84e1b96bb68fee5281cd
diff --git a/ast_java.h b/ast_java.h
index 7f6b9dc..b9c898e 100644
--- a/ast_java.h
+++ b/ast_java.h
@@ -161,10 +161,10 @@
 {
     Variable* lvalue;
     Expression* rvalue;
-    Type* cast;
+    const Type* cast;
 
     Assignment(Variable* lvalue, Expression* rvalue);
-    Assignment(Variable* lvalue, Expression* rvalue, Type* cast);
+    Assignment(Variable* lvalue, Expression* rvalue, const Type* cast);
     virtual ~Assignment();
     virtual void Write(CodeWriter* to) const;
 };