getResult() should not try to compute for non-Dart files.

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 584 matching lines @@
     // successfully imports file:///my_pkg/test/d.dart, causing y to have an
     // inferred type of String.
     {
       AnalysisResult result = await driver.getResult(b);
       List<AnalysisError> errors = result.errors;
       expect(errors, hasLength(1));
       expect(errors[0].errorCode, StaticTypeWarningCode.INVALID_ASSIGNMENT);
     }
   }
 
+  test_getResult_notDartFile() async {
+    var path = _p('/test/lib/test.txt');
+    provider.newFile(path, 'foo bar');
+
+    AnalysisResult result = await driver.getResult(path);
+    expect(result, isNull);
+  }
+
   test_getResult_sameFile_twoUris() async {
     var a = _p('/test/lib/a.dart');
     var b = _p('/test/lib/b.dart');
     var c = _p('/test/test/c.dart');
     provider.newFile(a, 'class A<T> {}');
     provider.newFile(
         b,
         r'''
 import 'a.dart';
 var VB = new A<int>();
@@ skipping 618 matching lines @@
     await _completer.future;
     _completer = new Completer<Null>();
   }
 
   void notify() {
     if (!_completer.isCompleted) {
       _completer.complete(null);
     }
   }
 }