TestUtils (flutter/packages) should be deleted/tests refactored

[matanlurey]

The camera_android plugin makes use of a TestUtils class with the following behavior:

public class TestUtils {
  public static <T> void setPrivateField(T instance, String fieldName, Object newValue) {
    try {
      Field field = instance.getClass().getDeclaredField(fieldName);
      field.setAccessible(true);
      field.set(instance, newValue);
    } catch (Exception e) {
      Assert.fail("Unable to mock private field: " + fieldName);
    }
  }

  public static <T> Object getPrivateField(T instance, String fieldName) {
    try {
      Field field = instance.getClass().getDeclaredField(fieldName);
      field.setAccessible(true);
      return field.get(instance);
    } catch (Exception e) {
      Assert.fail("Unable to mock private field: " + fieldName);
      return null;
    }
  }
}

This is ... not a good pattern. If the field needs to be made public, it should be made public (or package-private) and @VisibleForTesting used. That pattern itself should be kept to a minimum, and instead the APIs should have good inherent testability/make use of patterns like dependency injection.

This is blocking my work on flutter/packages#6461, and while I could work around it, I'd be spending a reasonable amount of time working around a bad pattern. If @reidbaker and team are on board, I'd rather spend my time helping you all delete this pattern.

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