Issue 28149. Don't include non-Dart file into FileState.

pkg/analyzer/test/src/dart/analysis/driver_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.driver;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1104 matching lines @@
 ''';
     addTestFile(content);
 
     CompilationUnitElement unitElement = await driver.getUnitElement(testFile);
     expect(unitElement, isNotNull);
     expect(unitElement.source.fullName, testFile);
     expect(unitElement.functions.map((c) => c.name),
         unorderedEquals(['foo', 'main']));
   }
 
+  test_getUnitElement_notDart() async {
+    CompilationUnitElement unit = await driver.getUnitElement('foo.txt');
+    expect(unit, isNull);
+  }
+
   test_hasFilesToAnalyze() async {
     // No files yet, nothing to analyze.
     expect(driver.hasFilesToAnalyze, isFalse);
 
     // Add a new file, it should be analyzed.
     addTestFile('main() {}', priority: false);
     expect(driver.hasFilesToAnalyze, isTrue);
 
     // Wait for idle, nothing to do.
     await _waitForIdle();
@@ skipping 659 matching lines @@
   String _p(String path) => provider.convertPath(path);
 
   Future<Null> _waitForIdle() async {
     await idleStatusMonitor.signal;
   }
 
   static String _md5(String content) {
     return hex.encode(md5.convert(UTF8.encode(content)).bytes);
   }
 }