Fix a crash in graoh builder where a throw node is added in an expression tree by the parser: allow…

runtime/vm/flow_graph_builder.h

Index: runtime/vm/flow_graph_builder.h
===================================================================
--- runtime/vm/flow_graph_builder.h	(revision 6338)
+++ runtime/vm/flow_graph_builder.h	(working copy)
@@ -163,6 +163,8 @@
                             int start_index,
                             Value* alloc_value);
 
+  void BuildThrowNode(ThrowNode* node);
+
  private:
   // Specify a computation as the final result.  Adds a Do instruction to
   // the graph, but normally overridden in subclasses.
@@ -201,6 +203,7 @@
   virtual void VisitBinaryOpNode(BinaryOpNode* node);
   virtual void VisitConditionalExprNode(ConditionalExprNode* node);
   virtual void VisitLoadLocalNode(LoadLocalNode* node);
+  virtual void VisitThrowNode(ThrowNode* node);
 
   Value* value() const { return value_; }