Update CI config for Flutter 2 (#3674)

Includes cleanup to simplify our setup. Major changes:
- Eliminate the NNBD plugin filtering for stable.
- Remove the temporarily-added beta branch testing.
- Enable Linux, macOS, and web on stable (Windows is LUCI-based)
- Combine the two different macOS matrix configurations now that they
  are the same.
- Combine the two different Linux matrix configurations by using a single
  Dockerfile (which now also includes clang-format)
  - The web integration smoke test temporarily still uses the old Dockerfile,
    now renamed, because the driver installer script doesn't support
    Chrome 89 yet.
- Move most of the Linux tasks to lower-CPU machines to allow more
  tasks to run in parallel without hitting the community limit.
- Reorder the tasks slightly and give them comments to identify
  platform groupings
- Enabled web "build all plugins together" and "build all examples"
  tests

script/tool/test/build_examples_command_test.dart

@@ -201,7 +201,7 @@ void main() {
         output,
         orderedEquals(<String>[
           '\nBUILDING macOS for $packageName',
-          '\macOS is not supported by this plugin',
+          'macOS is not supported by this plugin',
           '\n\n',
           'All builds successful!',
         ]),
@@ -213,6 +213,7 @@ void main() {
       expect(processRunner.recordedCalls, orderedEquals(<ProcessCall>[]));
       cleanupPackages();
     });
+
     test('building for macos', () async {
       createFakePlugin('plugin',
           withExtraFiles: <List<String>>[
@@ -244,14 +245,81 @@ void main() {
       expect(
           processRunner.recordedCalls,
           orderedEquals(<ProcessCall>[
-            ProcessCall(flutterCommand, <String>['pub', 'get'],
-                pluginExampleDirectory.path),
             ProcessCall(flutterCommand, <String>['build', 'macos'],
                 pluginExampleDirectory.path),
           ]));
       cleanupPackages();
     });
 
+    test('building for web with no implementation results in no-op', () async {
+      createFakePlugin('plugin', withExtraFiles: <List<String>>[
+        <String>['example', 'test'],
+      ]);
+
+      final Directory pluginExampleDirectory =
+          mockPackagesDir.childDirectory('plugin').childDirectory('example');
+
+      createFakePubspec(pluginExampleDirectory, isFlutter: true);
+
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['build-examples', '--no-ipa', '--web']);
+      final String packageName =
+          p.relative(pluginExampleDirectory.path, from: mockPackagesDir.path);
+
+      expect(
+        output,
+        orderedEquals(<String>[
+          '\nBUILDING web for $packageName',
+          'Web is not supported by this plugin',
+          '\n\n',
+          'All builds successful!',
+        ]),
+      );
+
+      print(processRunner.recordedCalls);
+      // Output should be empty since running build-examples --macos with no macos
+      // implementation is a no-op.
+      expect(processRunner.recordedCalls, orderedEquals(<ProcessCall>[]));
+      cleanupPackages();
+    });
+
+    test('building for web', () async {
+      createFakePlugin('plugin',
+          withExtraFiles: <List<String>>[
+            <String>['example', 'test'],
+            <String>['example', 'web', 'index.html'],
+          ],
+          isWebPlugin: true);
+
+      final Directory pluginExampleDirectory =
+          mockPackagesDir.childDirectory('plugin').childDirectory('example');
+
+      createFakePubspec(pluginExampleDirectory, isFlutter: true);
+
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['build-examples', '--no-ipa', '--web']);
+      final String packageName =
+          p.relative(pluginExampleDirectory.path, from: mockPackagesDir.path);
+
+      expect(
+        output,
+        orderedEquals(<String>[
+          '\nBUILDING web for $packageName',
+          '\n\n',
+          'All builds successful!',
+        ]),
+      );
+
+      print(processRunner.recordedCalls);
+      expect(
+          processRunner.recordedCalls,
+          orderedEquals(<ProcessCall>[
+            ProcessCall(flutterCommand, <String>['build', 'web'],
+                pluginExampleDirectory.path),
+          ]));
+      cleanupPackages();
+    });
+
     test(
         'building for Windows when plugin is not set up for Windows results in no-op',
         () async {