make checker_test, inferred_type_test easier to debug

pkg/analyzer/test/src/task/strong/strong_test_helper.dart

Index: pkg/analyzer/test/src/task/strong/strong_test_helper.dart
diff --git a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
index f386377d44db4e985e227a86622e95901db2ce4d..d18b4936b372d4ea8e57b6c852eda7545bdbb66a 100644
--- a/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
+++ b/pkg/analyzer/test/src/task/strong/strong_test_helper.dart
@@ -203,7 +203,7 @@ List<LibraryElement> reachableLibraries(LibraryElement start) {
 /// For example, to check that an assignment produces a warning about a boxing
 /// conversion, you can describe the test as follows:
 ///
-///     testChecker({
+///     testChecker('test feature', () => {
 ///       '/main.dart': '''
 ///           testMethod() {
 ///             dynamic x = /*warning:Box*/3;
@@ -211,9 +211,11 @@ List<LibraryElement> reachableLibraries(LibraryElement start) {
 ///       '''
 ///     });
 ///
-void testChecker(String name, Map<String, String> testFiles) {
+void testChecker(String name, Map<String, String> getTestFiles()) {
   test(name, () {
     AnalysisEngine.instance.processRequiredPlugins();
+
+    var testFiles = getTestFiles();
     expect(testFiles.containsKey('/main.dart'), isTrue,
         reason: '`/main.dart` is missing in testFiles');