initial checkin of SkSL compiler

src/sksl/ast/SkSLASTNode.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+ 
+#ifndef SKSL_ASTNODE
+#define SKSL_ASTNODE
+
+#include <memory>
+#include <string>
+
+namespace SkSL {
+
+/**
+ * Represents a node in the abstract syntax tree (AST). The AST is based directly on the parse tree;
+ * it is a parsed-but-not-yet-analyzed version of the program.
+ */
+struct ASTNode {
+    virtual ~ASTNode() {}
+        
+    virtual std::string description() const = 0;
+};
+
+} // namespace
+
+#endif