modify test_runner.dart for windows to fix test build errors

lib/web_ui/dev/test_runner.dart

@@ -60,6 +60,12 @@ class TestCommand extends Command<bool> {
 
     _copyTestFontsIntoWebUi();
     await _buildHostPage();
+    if (io.Platform.isWindows) {
+      // On Dart 2.7 or greater, it gives an error for not
+      // recognized "pub" version and asks for "pub" get.
+      // See: https://github.com/dart-lang/sdk/issues/39738
+      await _runPubGet();
+    }
 
     final List<FilePath> targets =
         this.targets.map((t) => FilePath.fromCwd(t)).toList();
@@ -162,7 +168,9 @@ class TestCommand extends Command<bool> {
           // Not a test file at all. Skip.
           continue;
         }
-        if(!path.split(testFilePath.relativeToWebUi).contains('golden_tests')) {
+        if (!path
+            .split(testFilePath.relativeToWebUi)
+            .contains('golden_tests')) {
           unitTestFiles.add(testFilePath);
         }
       }
@@ -179,6 +187,22 @@ class TestCommand extends Command<bool> {
     }
   }
 
+  Future<void> _runPubGet() async {
+    final int exitCode = await runProcess(
+      environment.pubExecutable,
+      <String>[
+        'get',
+      ],
+      workingDirectory: environment.webUiRootDir.path,
+    );
+
+    if (exitCode != 0) {
+      io.stderr
+          .writeln('Failed to run pub get. Exited with exit code $exitCode');
+      io.exit(1);
+    }
+  }
+
   Future<void> _buildHostPage() async {
     final String hostDartPath = path.join('lib', 'static', 'host.dart');
     final io.File hostDartFile = io.File(path.join(

lib/web_ui/dev/utils.dart

@@ -43,6 +43,9 @@ Future<int> runProcess(
     executable,
     arguments,
     workingDirectory: workingDirectory,
+    // Running the process in a system shell for Windows. Otherwise
+    // the process is not able to get Dart from path.
+    runInShell: io.Platform.isWindows,
     mode: io.ProcessStartMode.inheritStdio,
   );
   final int exitCode = await process.exitCode;