Added basic validation tests to graphql_schema

.idea/runConfigurations/tests_in_validation_test_dart.xml

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+<component name="ProjectRunConfigurationManager">
+  <configuration default="false" name="tests in validation_test.dart" type="DartTestRunConfigurationType" factoryName="Dart Test" singleton="true" nameIsGenerated="true">
+    <option name="filePath" value="$PROJECT_DIR$/graphql_schema/test/validation_test.dart" />
+    <option name="testRunnerOptions" value="-j4" />
+    <method />
+  </configuration>
+</component>

graphql_schema/test/validation_test.dart

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+import 'package:graphql_schema/graphql_schema.dart';
+import 'package:test/test.dart';
+
+void main() {
+  var pokemonType = objectType('Pokémon', fields: [
+    field('name', type: graphQLString.nonNullable()),
+  ]);
+
+  var isValidPokemon = predicate(
+      (x) =>
+          pokemonType.validate('@root', x as Map<String, dynamic>).successful,
+      'is a valid Pokémon');
+
+  var throwsATypeError =
+      throwsA(predicate((x) => x is TypeError, 'is a type error'));
+
+  test('mismatched scalar type', () {
+    expect(() => pokemonType.validate('@root', {'name': 24}), throwsATypeError);
+  });
+
+  test('empty passed for non-nullable', () {
+    expect(<String, dynamic>{}, isNot(isValidPokemon));
+  });
+
+  test('null passed for non-nullable', () {
+    expect({'name': null}, isNot(isValidPokemon));
+  });
+
+  test('rejects extraneous fields', () {
+    expect({'name': 'Vulpix', 'foo': 'bar'}, isNot(isValidPokemon));
+  });
+}

graphql_server/test/common.dart

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+import 'package:graphql_schema/graphql_schema.dart';
+import 'package:test/test.dart';
+
+final Matcher throwsAGraphQLException =
+    throwsA(predicate((x) => x is GraphQLException, 'is a GraphQL exception'));