Turned on SkSL->GLSL compiler

src/sksl/ast/SkSLASTIndexSuffix.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_ASTINDEXSUFFIX
 #define SKSL_ASTINDEXSUFFIX
 
 #include "SkSLASTExpression.h"
 #include "SkSLASTSuffix.h"
 
 namespace SkSL {
 
 /**
- * A bracketed expression, as in '[0]', indicating an array access. 
+ * A bracketed expression, as in '[0]', indicating an array access. Empty brackets (as occur in
+ * 'float[](5, 6)' are represented with a null fExpression.
  */
 struct ASTIndexSuffix : public ASTSuffix {
+    ASTIndexSuffix(Position position) 
+    : INHERITED(position, ASTSuffix::kIndex_Kind)
+    , fExpression(nullptr) {}
+
     ASTIndexSuffix(std::unique_ptr<ASTExpression> expression) 
-    : INHERITED(expression->fPosition, ASTSuffix::kIndex_Kind)
+    : INHERITED(expression ? expression->fPosition : Position(), ASTSuffix::kIndex_Kind)
     , fExpression(std::move(expression)) {}
 
     std::string description() const override {
-        return "[" + fExpression->description() + "]";
+        if (fExpression) {
+            return "[" + fExpression->description() + "]";
+        } else {
+            return "[]";
+        }
     }
 
+    // may be null
     std::unique_ptr<ASTExpression> fExpression;
 
     typedef ASTSuffix INHERITED;
 };
 
 } // namespace
 
 #endif