Bump analyzer to 2.4.0

[GZGavinZhao]

This makes pageloader compatible with AngularDart 7+.

However, currently this adds two more errors to the test:

Before:

01:05 +376 -1: test/webdriver_typing_test.dart: clear with focus and blur with empty text without focusBefore and blurAfter [E]                                                                                                              
  Expected: <1>
    Actual: <0>
  
  package:test_api            expect
  test/src/typing.dart 147:9  runTests.<fn>.<fn>.<fn>

Now:

01:09 +396 -1: test/webdriver_typing_test.dart: clear with focus and blur with empty text without focusBefore and blurAfter [E]                                                                                                              
  Expected: <0>
    Actual: <1>
  
  package:test_api            expect
  test/src/typing.dart 153:9  runTests.<fn>.<fn>.<fn>
  
01:12 +404 -2: test/webdriver_typing_test.dart: clear with focus and blur with non-empty text without focusBefore and blurAfter [E]                                                                                                          
  Expected: <2>
    Actual: <1>
  
  package:test_api            expect
  test/src/typing.dart 177:9  runTests.<fn>.<fn>.<fn>
  
01:22 +446 -3: test/webdriver_typing_null_safety_test.dart: typing with focus and blur without focusBefore and blurAfter [E]                                                                                                                 
  Expected: <1>
    Actual: <0>
  
  package:test_api                       expect
  test/src_null_safety/typing.dart 98:7  runTests.<fn>.<fn>
  
01:27 +459 -3: Some tests failed. 

[GZGavinZhao]

Oh that was because I ran test directly with dart test.

If I use ./tool/travis.sh, the test suite passes with no errors.