add use_test_throws_matchers

[a14n]

Use the throwsA matcher instead of try-catch with fail().

BAD:

// sync code
try {
  someSyncFunctionThatThrows();
  fail('expected Error');
} on Error catch (error) {
  expect(error.message, contains('some message'));
}
// async code
try {
  await someAsyncFunctionThatThrows();
  fail('expected Error');
} on Error catch (error) {
  expect(error.message, contains('some message'));
}

GOOD:

// sync code
expect(
  () => someSyncFunctionThatThrows(),
  throwsA(isA<Error>().having((Error error) => error.message, 'message', contains('some message'))),
);
// async code
await expectLater(
  () => someAsyncFunctionThatThrows(),
  throwsA(isA<Error>().having((Error error) => error.message, 'message', contains('some message'))),
);

[coveralls]

Coverage decreased (-0.3%) to 94.084% when pulling 8849c1f on a14n:use_test_throws_matchers into bf8de84 on dart-lang:master.

[bwilkerson]

Perhaps we could highlight the name 'fail' rather than the whole try statement.