Change ResourceUriResolver to accept any file URI.

We don't really need to check that the corresponding resource is
a file. Now it is a file, or a directory, the moment later it is not.
We will try to read it, if we can - fine, if not - it does not exist.

This makes my Flutter analysis benchmark about 3% faster.

R=brianwilkerson@google.com

Change-Id: I955be276c7a694a408c70f8f2d47dce75594ebc8
Reviewed-on: https://dart-review.googlesource.com/77243
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/file_system/file_system.dart

@@ -27,11 +27,8 @@ class ResourceUriResolver extends UriResolver {
       return null;
     }
     String path = fileUriToNormalizedPath(_provider.pathContext, uri);
-    Resource resource = _provider.getResource(path);
-    if (resource is File) {
-      return resource.createSource(actualUri ?? uri);
-    }
-    return null;
+    File file = _provider.getFile(path);
+    return file.createSource(actualUri ?? uri);
   }
 
   @override

pkg/analyzer/test/file_system/resource_uri_resolver_test.dart

@@ -42,7 +42,9 @@ class ResourceUriResolverTest {
   void test_resolveAbsolute_folder() {
     var uri = provider.pathContext.toUri(provider.convertPath('/folder'));
     Source source = resolver.resolveAbsolute(uri);
-    expect(source, isNull);
+    expect(source, isNotNull);
+    expect(source.exists(), isFalse);
+    expect(source.fullName, provider.convertPath('/folder'));
   }
 
   void test_resolveAbsolute_notFile_dartUri() {

pkg/analyzer/test/src/summary/resynthesize_common.dart

@@ -7417,9 +7417,9 @@ unit: b.dart
     var library = await checkLibrary('library my.lib; part "foo/";');
     checkElementText(library, r'''
 library my.lib;
-part '<unresolved>';
+part '';
 --------------------
-unit: null
+unit: foo
 
 ''');
   }

pkg/analyzer/test/src/task/dart_test.dart

@@ -895,7 +895,7 @@ library lib;
 part '$invalidUri';
 '''
     });
-    expect(libraryElement.parts, isEmpty);
+    expect(libraryElement.parts, hasLength(1));
   }
 
   test_perform_isLaunchable_inDefiningUnit() {