[tools] fix version check command not working for new packages (flutter#3818)

Author: Chris Yang

script/tool/lib/src/version_check_command.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:async';
-import 'dart:io' as io;
 
 import 'package:meta/meta.dart';
 import 'package:file/file.dart';
@@ -121,6 +120,7 @@ class VersionCheckCommand extends PluginCommand {
       if (masterVersion == null) {
         print('${indentation}Unable to find pubspec in master. '
             'Safe to ignore if the project is new.');
+        continue;
       }
 
       if (masterVersion == headVersion) {

script/tool/test/version_check_test.dart

@@ -61,6 +61,9 @@ void main() {
         } else if (invocation.positionalArguments[0][0] == 'show') {
           final String response =
               gitShowResponses[invocation.positionalArguments[0][1]];
+          if (response == null) {
+            throw const io.ProcessException('git', <String>['show']);
+          }
           when<String>(mockProcessResult.stdout as String).thenReturn(response);
         } else if (invocation.positionalArguments[0][0] == 'merge-base') {
           when<String>(mockProcessResult.stdout as String).thenReturn('abc123');
@@ -150,6 +153,23 @@ void main() {
       );
     });
 
+    test('allows valid version for new package.', () async {
+      createFakePlugin('plugin', includeChangeLog: true, includeVersion: true);
+      gitDiffResponse = 'packages/plugin/pubspec.yaml';
+      gitShowResponses = <String, String>{
+        'HEAD:packages/plugin/pubspec.yaml': 'version: 1.0.0',
+      };
+      final List<String> output =
+          await runCapturingPrint(runner, <String>['version-check']);
+
+      expect(
+        output,
+        containsAllInOrder(<String>[
+          'No version check errors found!',
+        ]),
+      );
+    });
+
     test('denies invalid version without explicit base-sha', () async {
       createFakePlugin('plugin', includeChangeLog: true, includeVersion: true);
       gitDiffResponse = 'packages/plugin/pubspec.yaml';