Fix MockSdk.fromFileUri() implementations.

pkg/analysis_server/test/mock_sdk.dart

Index: pkg/analysis_server/test/mock_sdk.dart
diff --git a/pkg/analysis_server/test/mock_sdk.dart b/pkg/analysis_server/test/mock_sdk.dart
index 9027e6d05e6b5d9045cbc842e193b92e8208a6e0..ab40d919541f0d7e61ee8137c7f2b8518d615565 100644
--- a/pkg/analysis_server/test/mock_sdk.dart
+++ b/pkg/analysis_server/test/mock_sdk.dart
@@ -334,28 +334,26 @@ const Map<String, LibraryInfo> libraries = const {
 
   @override
   Source fromFileUri(Uri uri) {
-    String filePath = uri.path;
-    String libPath = '/lib';
-    if (!filePath.startsWith("$libPath/")) {
-      return null;
-    }
-    for (SdkLibrary library in LIBRARIES) {
-      String libraryPath = library.path;
-      if (filePath.replaceAll('\\', '/') == libraryPath) {
+    String filePath = provider.pathContext.fromUri(uri);
+    for (SdkLibrary library in sdkLibraries) {
+      String libraryPath = provider.convertPath(library.path);
+      if (filePath == libraryPath) {
         try {
-          resource.File file = provider.getResource(uri.path);
+          resource.File file = provider.getResource(filePath);
           Uri dartUri = Uri.parse(library.shortName);
           return file.createSource(dartUri);
         } catch (exception) {
           return null;
         }
       }
-      if (filePath.startsWith("$libraryPath/")) {
-        String pathInLibrary = filePath.substring(libraryPath.length + 1);
-        String path = '${library.shortName}/$pathInLibrary';
+      String libraryRootPath = provider.pathContext.dirname(libraryPath) +
+          provider.pathContext.separator;
+      if (filePath.startsWith(libraryRootPath)) {
+        String pathInLibrary = filePath.substring(libraryRootPath.length);
+        String uriStr = '${library.shortName}/$pathInLibrary';
         try {
-          resource.File file = provider.getResource(uri.path);
-          Uri dartUri = new Uri(scheme: 'dart', path: path);
+          resource.File file = provider.getResource(filePath);
+          Uri dartUri = Uri.parse(uriStr);
           return file.createSource(dartUri);
         } catch (exception) {
           return null;