Build the solution on Windows (#33528)

Switch from building Runner.vcxproj to Runner.sln on Windows, to allow
for multiple-project builds (e.g., separate plugin projects).
diff --git a/packages/flutter_tools/lib/src/windows/build_windows.dart b/packages/flutter_tools/lib/src/windows/build_windows.dart
index c31708b..eddd533 100644
--- a/packages/flutter_tools/lib/src/windows/build_windows.dart
+++ b/packages/flutter_tools/lib/src/windows/build_windows.dart
@@ -38,16 +38,16 @@
   );
 
   final String configuration = buildInfo.isDebug ? 'Debug' : 'Release';
-  final String projectPath = windowsProject.vcprojFile.path;
+  final String solutionPath = windowsProject.solutionFile.path;
   // Run the script with a relative path to the project using the enclosing
   // directory as the workingDirectory, to avoid hitting the limit on command
   // lengths in batch scripts if the absolute path to the project is long.
   final Process process = await processManager.start(<String>[
     buildScript,
     vcvarsScript,
-    fs.path.basename(projectPath),
+    fs.path.basename(solutionPath),
     configuration,
-  ], workingDirectory: fs.path.dirname(projectPath));
+  ], workingDirectory: fs.path.dirname(solutionPath));
   final Status status = logger.startProgress(
     'Building Windows application...',
     timeout: null,