[flutter_tools] Add --verify-only flag to flutter upgrade (#68866)

* Add --verify-only flag for flutter upgrade

packages/flutter_tools/lib/src/commands/upgrade.dart

@@ -38,6 +38,11 @@ class UpgradeCommand extends FlutterCommand {
         'working-directory',
         hide: true,
         help: 'Override the upgrade working directory for integration testing.'
+      )
+      ..addFlag(
+        'verify-only',
+        help: 'Verifies for any new flutter update, without fetching the update.',
+        negatable: false,
       );
   }
 
@@ -63,6 +68,7 @@ class UpgradeCommand extends FlutterCommand {
       flutterVersion: stringArg('working-directory') == null
         ? globals.flutterVersion
         : FlutterVersion(const SystemClock(), _commandRunner.workingDirectory),
+      verifyOnly: boolArg('verify-only'),
     );
   }
 }
@@ -78,6 +84,7 @@ class UpgradeCommandRunner {
     @required bool testFlow,
     @required GitTagVersion gitTagVersion,
     @required FlutterVersion flutterVersion,
+    @required bool verifyOnly,
   }) async {
     if (!continueFlow) {
       await runCommandFirstHalf(
@@ -85,6 +92,7 @@ class UpgradeCommandRunner {
         gitTagVersion: gitTagVersion,
         flutterVersion: flutterVersion,
         testFlow: testFlow,
+        verifyOnly: verifyOnly,
       );
     } else {
       await runCommandSecondHalf(flutterVersion);
@@ -97,12 +105,24 @@ class UpgradeCommandRunner {
     @required GitTagVersion gitTagVersion,
     @required FlutterVersion flutterVersion,
     @required bool testFlow,
+    @required bool verifyOnly,
   }) async {
     final String upstreamRevision = await fetchRemoteRevision();
     if (flutterVersion.frameworkRevision == upstreamRevision) {
       globals.printStatus('Flutter is already up to date on channel ${flutterVersion.channel}');
       globals.printStatus('$flutterVersion');
       return;
+    } else if (verifyOnly) {
+      globals.printStatus('A new version of Flutter is available on channel ${flutterVersion.channel}\n');
+      // TODO(fujino): use a [FlutterVersion] once that class supports arbitrary revisions.
+      globals.printStatus('The latest revision: $upstreamRevision', emphasis: true);
+      globals.printStatus('Your current version: ${flutterVersion.frameworkRevision}\n');
+      globals.printStatus('To upgrade now, run "flutter upgrade".');
+      if (flutterVersion.channel == 'stable') {
+        globals.printStatus('\nSee the announcement and release notes:');
+        globals.printStatus('https://flutter.dev/docs/development/tools/sdk/release-notes');
+      }
+      return;
     }
     if (!force && gitTagVersion == const GitTagVersion.unknown()) {
       // If the commit is a recognized branch and not master,

packages/flutter_tools/test/commands.shard/permeable/upgrade_test.dart

@@ -54,6 +54,7 @@ void main() {
         testFlow: false,
         gitTagVersion: const GitTagVersion.unknown(),
         flutterVersion: flutterVersion,
+        verifyOnly: false,
       );
       expect(result, throwsToolExit());
       expect(processManager.hasRemainingExpectations, isFalse);
@@ -69,6 +70,7 @@ void main() {
         testFlow: false,
         gitTagVersion: gitTagVersion,
         flutterVersion: flutterVersion,
+        verifyOnly: false,
       );
       expect(result, throwsToolExit());
       expect(processManager.hasRemainingExpectations, isFalse);
@@ -87,6 +89,7 @@ void main() {
         testFlow: false,
         gitTagVersion: gitTagVersion,
         flutterVersion: flutterVersion,
+        verifyOnly: false,
       );
       expect(await result, FlutterCommandResult.success());
       expect(testLogger.statusText, contains('Flutter is already up to date'));
@@ -96,6 +99,28 @@ void main() {
       Platform: () => fakePlatform,
     });
 
+    testUsingContext('Correctly provides upgrade version on verify only', () async {
+      const String revision = 'abc123';
+      when(flutterVersion.frameworkRevision).thenReturn(revision);
+      fakeCommandRunner.alreadyUpToDate = false;
+      final Future<FlutterCommandResult> result = fakeCommandRunner.runCommand(
+        force: false,
+        continueFlow: false,
+        testFlow: false,
+        gitTagVersion: gitTagVersion,
+        flutterVersion: flutterVersion,
+        verifyOnly: true,
+      );
+      expect(await result, FlutterCommandResult.success());
+      expect(testLogger.statusText, contains('A new version of Flutter is available'));
+      expect(testLogger.statusText, contains(fakeCommandRunner.remoteRevision));
+      expect(testLogger.statusText, contains(revision));
+      expect(processManager.hasRemainingExpectations, isFalse);
+    }, overrides: <Type, Generator>{
+      ProcessManager: () => processManager,
+      Platform: () => fakePlatform,
+    });
+
     testUsingContext('fetchRemoteRevision returns revision if git succeeds', () async {
       const String revision = 'abc123';
 
@@ -255,6 +280,7 @@ void main() {
           testFlow: false,
           gitTagVersion: const GitTagVersion.unknown(),
           flutterVersion: flutterVersion,
+          verifyOnly: false,
         );
         expect(await result, FlutterCommandResult.success());
         expect(processManager.hasRemainingExpectations, isFalse);
@@ -272,6 +298,7 @@ void main() {
           testFlow: false,
           gitTagVersion: gitTagVersion,
           flutterVersion: flutterVersion,
+          verifyOnly: false,
         );
         expect(await result, FlutterCommandResult.success());
         expect(processManager.hasRemainingExpectations, isFalse);
@@ -287,6 +314,7 @@ void main() {
           testFlow: false,
           gitTagVersion: gitTagVersion,
           flutterVersion: flutterVersion,
+          verifyOnly: false,
         );
         expect(await result, FlutterCommandResult.success());
         expect(processManager.hasRemainingExpectations, isFalse);