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/lib/src/format_command.dart

@@ -22,10 +22,10 @@ class FormatCommand extends PluginCommand {
     FileSystem fileSystem, {
     ProcessRunner processRunner = const ProcessRunner(),
   }) : super(packagesDir, fileSystem, processRunner: processRunner) {
-    argParser.addFlag('travis', hide: true);
+    argParser.addFlag('fail-on-change', hide: true);
     argParser.addOption('clang-format',
         defaultsTo: 'clang-format',
-        help: 'Path to executable of clang-format v5.');
+        help: 'Path to executable of clang-format.');
   }
 
   @override
@@ -46,7 +46,7 @@ class FormatCommand extends PluginCommand {
     await _formatJava(googleFormatterPath);
     await _formatCppAndObjectiveC();
 
-    if (argResults['travis']) {
+    if (argResults['fail-on-change']) {
       final bool modified = await _didModifyAnything();
       if (modified) {
         throw ToolExit(1);