Fix MockSdk.fromFileUri() implementations.

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 4b15a90e710031b7eecdd7d38b9ae4d7b952db87..ea2cdc27f9e3d3b7b5f823160f3a2b86726f7e68 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -485,30 +485,29 @@ class MockSdk implements DartSdk {
 
   @override
   Source fromFileUri(Uri uri) {
-    String filePath = uri.path;
-    String libPath = '$sdkRoot/lib';
-    if (!filePath.startsWith("$libPath/")) {
+    String filePath = provider.pathContext.fromUri(uri);
+    if (!filePath.startsWith(provider.convertPath('$sdkRoot/lib/'))) {
       return null;
     }
     for (SdkLibrary library in sdkLibraries) {
-      String libraryPath = library.path;
-      if (filePath.replaceAll('\\', '/') == libraryPath) {
+      String libraryPath = provider.convertPath(library.path);
+      if (filePath == libraryPath) {
         try {
-          resource.File file =
-              provider.getResource(provider.convertPath(filePath));
+          resource.File file = provider.getResource(filePath);
           Uri dartUri = Uri.parse(library.shortName);
           return file.createSource(dartUri);
         } catch (exception) {
           return null;
         }
       }
-      if (filePath.startsWith("$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(provider.convertPath(filePath));
-          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;