Eliminate deprecation warning

'SpecParser.java' has always been using an `ANTLRFileStream`, but this
class has been deprecated at this time. Changing the code to use
`CharStreams.fromFileName(...)`.

Change-Id: I35fce3b381ac2e819794c08faef7f97c4c3bea50
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/444840
Commit-Queue: Erik Ernst <eernst@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

tools/spec_parser/SpecParser.java

@@ -122,7 +122,7 @@ public class SpecParser {
           continue;
       }
       if (verbose) System.err.println("Parsing file: " + filePath);
-      DartLexer lexer = new DartLexer(new ANTLRFileStream(filePath));
+      DartLexer lexer = new DartLexer(CharStreams.fromFileName(filePath));
       DartParser parser = new DartParser(new CommonTokenStream(lexer));
       ANTLRErrorListener errorListener = new DartErrorListener();
       lexer.addErrorListener(errorListener);