Convert comment_processable_test to use MemoryResourceProvider. (#2324)

Convert comment_processable_test to use MemoryResourceProvider.

Add a few tests for @example.
Remove a few unnecessary Fake classes.

Author: srawlins

lib/src/model/comment_processable.dart

@@ -208,6 +208,8 @@ mixin CommentProcessable on Documentable, Warnable, Locatable, SourceCodeMixin {
       } else {
         var filePath = element.source.fullName.substring(dirPath.length + 1);
 
+        // TODO(srawlins): If a file exists at the location without the
+        // appended 'md' extension, note this.
         warn(PackageWarning.missingExampleFile,
             message: '${fragmentFile.path}; path listed at $filePath');
       }

lib/src/package_meta.dart

@@ -286,13 +286,9 @@ class _FilePackageMeta extends PubPackageMeta {
 
   _FilePackageMeta(Folder dir, ResourceProvider resourceProvider)
       : super(dir, resourceProvider) {
-    var f = resourceProvider
-        .getFile(resourceProvider.pathContext.join(dir.path, 'pubspec.yaml'));
-    if (f.exists) {
-      _pubspec = loadYaml(f.readAsStringSync());
-    } else {
-      _pubspec = <dynamic, dynamic>{};
-    }
+    var pubspec = dir.getChildAssumingFile('pubspec.yaml');
+    assert(pubspec.exists);
+    _pubspec = loadYaml(pubspec.readAsStringSync());
   }
 
   bool _setHostedAt = false;