make checker_test, inferred_type_test easier to debug

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

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 library analyzer.test.src.task.strong.strong_test_helper;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 185 matching lines @@
 ///   * a file named `/main.dart` exists in [testFiles].
 ///   * all expected failures are listed in the source code using comments
 ///   immediately in front of the AST node that should contain the error.
 ///   * errors are formatted as a token `level:Type`, where `level` is the
 ///   logging level were the error would be reported at, and `Type` is the
 ///   concrete subclass of [StaticInfo] that denotes the error.
 ///
 /// 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;
 ///           }
 ///       '''
 ///     });
 ///
-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');
 
     var provider = new MemoryResourceProvider();
     testFiles.forEach((key, value) {
       var scheme = 'package:';
       if (key.startsWith(scheme)) {
         key = '/packages/${key.substring(scheme.length)}';
       }
       provider.newFile(key, value);
@@ skipping 289 matching lines @@
   UriKind get uriKind => UriKind.DART_URI;
 
   bool exists() => true;
 
   Source resolveRelative(Uri relativeUri) =>
       throw new UnsupportedError('not expecting relative urls in dart: mocks');
 
   Uri resolveRelativeUri(Uri relativeUri) =>
       throw new UnsupportedError('not expecting relative urls in dart: mocks');
 }