Migrate flutter analyze config to options.

More groundwork to get `analyze` and IDE output to agree.

See: https://github.com/dart-lang/sdk/issues/25723

packages/flutter_tools/.analysis_options

@@ -5,6 +5,9 @@
 # For a list of lints, see: http://dart-lang.github.io/linter/lints/
 
 analyzer:
+  language:
+    enableSuperMixins: true
+  strong-mode: false #TODO(pq): enable once strong-mode runs clean
   errors:
     # we allow overriding fields (if they use super, ideally...)
     strong_mode_invalid_field_override: ignore

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

@@ -283,10 +283,8 @@ class AnalyzeCommand extends FlutterCommand {
       sdkBinaryName('dartanalyzer'),
       // do not set '--warnings', since that will include the entire Dart SDK
       '--ignore-unrecognized-flags',
-      '--supermixin',
-      '--enable-strict-call-checks',
+      '--enable-strict-call-checks', //TODO(pq): migrate to options once supported (dart/sdk#25983)
       '--enable_type_checks',
-      '--strong',
       '--package-warnings',
       '--fatal-warnings',
       '--fatal-hints',