Bump to 3.0.0-alpha+3

[srawlins]

This release allows projects to conform to the same type standards as internal code.

The most common way this restricts users of mockito is the following:

class MockFile extends Mock implements File {}

var f = new MockFile();
// Old: bad code returns a String where a Future<String> is expected.
when(f.readAsString()).thenReturn('text'));

// But no runtime errors are thrown because the return value is merely awaited.
var text = await f.readAsString();

// New: good code answers with an async closure that returns a String.
when(f.readAsString()).thenAnswer((_) async => 'text');