Issue title: How does the first proto file import the message in the second file？
 Issue body: syntax = "proto3";
package com.mobuz;
import “AddressInfo”；
option java_package = "com.proto.user";
option java_outer_classname = "UserInfo";
message User{
string uid=1;
string uname=2;
}

syntax = "proto3";
package com.mobuz;
option java_package = "com.proto.user";
option java_outer_classname = "AddressInfo";

message Address{
string state=1;
string city=2;
}

How do I import the AddressInfo into UserInfo by js？
O(∩_∩)O


 Comments: 
 Comment 0: If the two proto files are in the same directory, you can directly reference it, or you should use the full name. 
 Comment 1: See the https://developers.google.com/protocol-buffers/docs/proto#other for importing other messages.
 Comment 2: I had the same problem as @yanxinyuan mentioned it . 
I have a Base.Proto file which include these messages :
`message ResultProto{
	bool succeed = 1;
}
message empty{}`
 and then I'm trying to use these messages within another protos, but I have got bellow errors :

`ResultProto" is not defined`
`empty is not defined`

is there any solution ?
{'source': 'GithubIssues', 'repo_name_with_owner': 'google/protobuf', 'issue_id': 268643672, 'repo_stars': 29630.0}