Update APIs to use new Google cloud paths.

client/flagpb/resolver.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package flagpb
 
-import "github.com/luci/luci-go/common/proto/google/descriptor"
+import (
+        "github.com/luci/luci-go/common/proto/google/descutil"
+
+        "google.golang.org/genproto/protobuf"
+)
 
 // Resolver resolves type names.
 type Resolver interface {
         // Resolve resolves a type name to
         // *descriptor.DescriptorProto or *descriptor.EnumDescriptorProto.
         Resolve(name string) interface{}
 }
 
 // NewResolver creates a resolver for all types in a file descriptor set.
 // Resolving time complexity is linear.
 func NewResolver(set *descriptor.FileDescriptorSet) Resolver {
         return &descriptorSetResolver{set}
 }
 
 type descriptorSetResolver struct {
         set *descriptor.FileDescriptorSet
 }
 
 func (r *descriptorSetResolver) Resolve(name string) interface{} {
-        _, o, _ := r.set.Resolve(name)
+        _, o, _ := descutil.Resolve(r.set, name)
         switch o := o.(type) {
         case *descriptor.DescriptorProto, *descriptor.EnumDescriptorProto:
                 return o
         default:
                 return nil
         }
 }