When running tests with --preview-dart-2 check for compilation errors. (#14724)

If compiler failed to produce Kernel binary then compile(...) returns
null to the caller. If we don't check for null we end up trying to
run file called "null" which causes a very confusing crash of the
flutter_tester.

packages/flutter_tools/lib/src/test/flutter_platform.dart

@@ -211,6 +211,11 @@ class _FlutterPlatform extends PlatformPlugin {
           packagesPath: PackageMap.globalPackagesPath,
         );
 
+        if (mainDart == null) {
+          controller.sink.addError(_getErrorMessage('Compilation failed', testPath, shellPath));
+          return null;
+        }
+
         // bundlePath needs to point to a folder with `platform.dill` file.
         final Directory tempBundleDirectory = fs.systemTempDirectory
             .createTempSync('flutter_bundle_directory');