Make MemoryResourceProvider.convertPath() work with partially posix paths.

pkg/analyzer/lib/file_system/memory_file_system.dart

Index: pkg/analyzer/lib/file_system/memory_file_system.dart
diff --git a/pkg/analyzer/lib/file_system/memory_file_system.dart b/pkg/analyzer/lib/file_system/memory_file_system.dart
index 42731be1b09940893addb31a4f68e197b71bcd2d..20f69e7260ee59dd0f2e8c3c18e04ec223e7c684 100644
--- a/pkg/analyzer/lib/file_system/memory_file_system.dart
+++ b/pkg/analyzer/lib/file_system/memory_file_system.dart
@@ -50,10 +50,11 @@ class MemoryResourceProvider implements ResourceProvider {
    * this class are never converted automatically.
    */
   String convertPath(String path) {
-    if (pathContext.style == pathos.windows.style &&
-        path.startsWith(pathos.posix.separator)) {
-      path = r'C:' +
-          path.replaceAll(pathos.posix.separator, pathos.windows.separator);
+    if (pathContext.style == pathos.windows.style) {
+      if (path.startsWith(pathos.posix.separator)) {
+        path = r'C:' + path;
+      }
+      path = path.replaceAll(pathos.posix.separator, pathos.windows.separator);
     }
     return path;
   }