[script/tool] Use 'dart pub' instead of deprecated 'pub' (#3991)

Author: mrhota

script/tool/lib/src/test_command.dart

@@ -66,14 +66,15 @@ class TestCommand extends PluginCommand {
         );
       } else {
         exitCode = await processRunner.runAndStream(
-          'pub',
-          <String>['get'],
+          'dart',
+          <String>['pub', 'get'],
           workingDir: packageDir,
         );
         if (exitCode == 0) {
           exitCode = await processRunner.runAndStream(
-            'pub',
+            'dart',
             <String>[
+              'pub',
               'run',
               if (enableExperiment.isNotEmpty)
                 '--enable-experiment=$enableExperiment',

script/tool/test/test_command_test.dart

@@ -94,10 +94,10 @@ void main() {
               'flutter',
               const <String>['test', '--color', '--enable-experiment=exp1'],
               plugin1Dir.path),
-          ProcessCall('pub', const <String>['get'], plugin2Dir.path),
+          ProcessCall('dart', const <String>['pub', 'get'], plugin2Dir.path),
           ProcessCall(
-              'pub',
-              const <String>['run', '--enable-experiment=exp1', 'test'],
+              'dart',
+              const <String>['pub', 'run', '--enable-experiment=exp1', 'test'],
               plugin2Dir.path),
         ]),
       );
@@ -149,10 +149,10 @@ void main() {
               'flutter',
               const <String>['test', '--color', '--enable-experiment=exp1'],
               plugin1Dir.path),
-          ProcessCall('pub', const <String>['get'], plugin2Dir.path),
+          ProcessCall('dart', const <String>['pub', 'get'], plugin2Dir.path),
           ProcessCall(
-              'pub',
-              const <String>['run', '--enable-experiment=exp1', 'test'],
+              'dart',
+              const <String>['pub', 'run', '--enable-experiment=exp1', 'test'],
               plugin2Dir.path),
         ]),
       );