go/ast: add missing handling of SendStmt to ast.Walk

R=adg
CC=golang-dev
https://golang.org/cl/4044048
diff --git a/src/pkg/go/ast/walk.go b/src/pkg/go/ast/walk.go
index d90c789..a77f8ee 100644
--- a/src/pkg/go/ast/walk.go
+++ b/src/pkg/go/ast/walk.go
@@ -195,6 +195,10 @@
 	case *ExprStmt:
 		Walk(v, n.X)
 
+	case *SendStmt:
+		Walk(v, n.Chan)
+		Walk(v, n.Value)
+
 	case *IncDecStmt:
 		Walk(v, n.X)