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

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

Index: pkg/analyzer/test/src/dart/analysis/driver_test.dart
diff --git a/pkg/analyzer/test/src/dart/analysis/driver_test.dart b/pkg/analyzer/test/src/dart/analysis/driver_test.dart
index 3b7d74d4a6b7d13c15632afd34a17b6934e71478..ec693f4982847b042be58429e4ad8f3a1e56bc5c 100644
--- a/pkg/analyzer/test/src/dart/analysis/driver_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/driver_test.dart
@@ -602,6 +602,14 @@ String z = "string";
     }
   }
 
+  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');