[tool] Allow importing packages with NEXT (flutter#3215)

Fixes an issue that showed up in the trial repo merge PR; if a pacakge
is new to the repository (i.e., has no git history in the parent
commit), it was being treated as a version bump, which caused failures
if NEXT was present. This adds a new state so that we can allow NEXT
during import.

script/tool/lib/src/version_check_command.dart

@@ -47,6 +47,9 @@ enum _CurrentVersionState {
   /// The version has changed, and the transition is invalid.
   invalidChange,
 
+  /// The package is new.
+  newPackage,
+
   /// There was an error determining the version state.
   unknown,
 }
@@ -216,6 +219,7 @@ class VersionCheckCommand extends PackageLoopingCommand {
         break;
       case _CurrentVersionState.validIncrease:
       case _CurrentVersionState.validRevert:
+      case _CurrentVersionState.newPackage:
         versionChanged = true;
         break;
       case _CurrentVersionState.invalidChange:
@@ -318,7 +322,7 @@ ${indentation}HTTP response: ${pubVersionFinderResponse.httpResponse.body}
           '${getBoolArg(_againstPubFlag) ? 'on pub server' : 'at git base'}.');
       logWarning(
           '${indentation}If this package is not new, something has gone wrong.');
-      return _CurrentVersionState.validIncrease; // Assume new, thus valid.
+      return _CurrentVersionState.newPackage;
     }
 
     if (previousVersion == currentVersion) {

script/tool/test/version_check_command_test.dart

@@ -525,6 +525,9 @@ void main() {
       const String version = '1.0.1';
       final RepositoryPackage plugin =
           createFakePlugin('plugin', packagesDir, version: version);
+      processRunner.mockProcessesForExecutable['git-show'] = <io.Process>[
+        MockProcess(stdout: 'version: 1.0.0'),
+      ];
 
       const String changelog = '''
 ## NEXT
@@ -538,7 +541,7 @@ void main() {
 
       bool hasError = false;
       final List<String> output = await runCapturingPrint(
-          runner, <String>['version-check', '--base-sha=main', '--against-pub'],
+          runner, <String>['version-check', '--base-sha=main'],
           errorHandler: (Error e) {
         expect(e, isA<ToolExit>());
         hasError = true;
@@ -559,6 +562,9 @@ void main() {
     test('fails if the version increases without replacing NEXT', () async {
       final RepositoryPackage plugin =
           createFakePlugin('plugin', packagesDir, version: '1.0.1');
+      processRunner.mockProcessesForExecutable['git-show'] = <io.Process>[
+        MockProcess(stdout: 'version: 1.0.0'),
+      ];
 
       const String changelog = '''
 ## NEXT
@@ -570,7 +576,7 @@ void main() {
 
       bool hasError = false;
       final List<String> output = await runCapturingPrint(
-          runner, <String>['version-check', '--base-sha=main', '--against-pub'],
+          runner, <String>['version-check', '--base-sha=main'],
           errorHandler: (Error e) {
         expect(e, isA<ToolExit>());
         hasError = true;
@@ -613,6 +619,30 @@ void main() {
       );
     });
 
+    // This handles imports of a package with a NEXT section.
+    test('allows NEXT for a new package', () async {
+      final RepositoryPackage plugin =
+          createFakePackage('a_package', packagesDir, version: '1.0.0');
+
+      const String changelog = '''
+## NEXT
+* Some changes that should be listed in the next release.
+## 1.0.0
+* Some other changes.
+''';
+      plugin.changelogFile.writeAsStringSync(changelog);
+
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['version-check', '--base-sha=main']);
+      expect(
+        output,
+        containsAllInOrder(<Matcher>[
+          contains('Unable to find previous version at git base'),
+          contains('Found NEXT; validating next version in the CHANGELOG'),
+        ]),
+      );
+    });
+
     test(
         'fails gracefully if the version headers are not found due to using the wrong style',
         () async {