Define a constant for the comments channel number

tool/src/org/antlr/v4/parse/ANTLRLexer.g

@@ -150,6 +150,8 @@ import org.antlr.v4.tool.*;
 
 
 @members {
+	public static final int COMMENTS_CHANNEL = 2;
+
     public CommonTokenStream tokens; // track stream we push to; need for context info
     public boolean isLexerRule = false;
 
@@ -265,7 +267,7 @@ COMMENT
          // writing a formatter then you will want to preserve the comments off
          // channel, but could just skip and save token space if not.
          //
-         $channel=2;  // Comments are on channel 2
+         $channel=COMMENTS_CHANNEL;
        }
     ;