Add back main() methods to benchmark benches. (#156083)

Dart performance suite needs this since it builds and launches those flutter benchmarks independently

Cf. https://github.com/flutter/flutter/pull/154446

dev/benchmarks/microbenchmarks/lib/geometry/matrix_utils_transform_bench.dart

@@ -136,3 +136,10 @@ Future<void> execute()  async {
   );
   printer.printToStdout();
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}

dev/benchmarks/microbenchmarks/lib/geometry/rrect_contains_bench.dart

@@ -28,3 +28,10 @@ Future<void> execute() async {
   );
   printer.printToStdout();
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}

dev/benchmarks/microbenchmarks/lib/gestures/gesture_detector_bench.dart

@@ -49,3 +49,10 @@ Future<void> execute() async {
   );
   printer.printToStdout();
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}

dev/benchmarks/microbenchmarks/lib/gestures/velocity_tracker_bench.dart

@@ -57,3 +57,10 @@ Future<void> execute() async {
 
   printer.printToStdout();
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}

dev/benchmarks/microbenchmarks/lib/language/sync_star_bench.dart

@@ -89,3 +89,10 @@ int sumIterable(Iterable<int> values) {
   }
   return result;
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}

dev/benchmarks/microbenchmarks/lib/language/sync_star_semantics_bench.dart

@@ -117,3 +117,10 @@ Iterable<InlineSpanSemanticsInformation> combineSemanticsInfoList(List<InlineSpa
   assert(workingLabel != null);
   return result;
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}

dev/benchmarks/microbenchmarks/lib/stocks/animation_bench.dart

@@ -90,3 +90,10 @@ Future<void> execute(BenchmarkingBinding binding) async {
   }
   printer.printToStdout();
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute(BenchmarkingBinding());
+}

dev/benchmarks/microbenchmarks/lib/stocks/build_bench.dart

@@ -65,3 +65,10 @@ Future<void> execute() async {
   );
   printer.printToStdout();
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}

dev/benchmarks/microbenchmarks/lib/stocks/layout_bench.dart

@@ -60,3 +60,10 @@ Future<void> execute() async {
   );
   printer.printToStdout();
 }
+
+//
+//  Note that the benchmark is normally run by benchmark_collection.dart.
+//
+Future<void> main() async {
+  return execute();
+}