Update devfs_test to use MemoryFileSystem (#8811)

packages/flutter_driver/pubspec.yaml

@@ -8,7 +8,7 @@ environment:
   sdk: '>=1.19.0 <2.0.0'
 
 dependencies:
-  file: 2.3.1
+  file: 2.3.2
   json_rpc_2: '^2.0.0'
   matcher: '>=0.12.0 <1.0.0'
   meta: ^1.0.4

packages/flutter_tools/lib/src/devfs.dart

@@ -551,7 +551,7 @@ class DevFS {
 
     for (String packageName in packageMap.map.keys) {
       final Uri packageUri = packageMap.map[packageName];
-      final String packagePath = packageUri.toFilePath();
+      final String packagePath = fs.path.fromUri(packageUri);
       final Directory packageDirectory = fs.directory(packageUri);
       Uri directoryUriOnDevice = fs.path.toUri(fs.path.join('packages', packageName) + fs.path.separator);
       bool packageExists;

packages/flutter_tools/pubspec.yaml

@@ -12,7 +12,7 @@ dependencies:
   args: ^0.13.4
   coverage: ^0.8.0
   crypto: '>=1.1.1 <3.0.0'
-  file: 2.3.1
+  file: 2.3.2
   http: ^0.11.3+12
   intl: '>=0.14.0 <0.15.0'
   json_rpc_2: ^2.0.0