[BUG] ecj vs. javac difference: @NonNull + @Builder will not generate the nullcheck on the 'builder-setter' in ecj only

[rzwitserloot]

Describe the bug
Both @SuperBuilder and @Builder suffer from this (reproduced in lombok v1.18.6): When producing the builder-setter for a @NonNull-marked field, in javac the builder-setter has an explicit nullcheck, but the class file compiled with ecj does not.

To Reproduce

@lombok.Builder
public class Test {
    @lombok.NonNull String id;
}

and then ecj -cp lombok.jar Test.java, then javap -private -c Test and note how there's no null check on the Test.TestBuilder.id(String id) field, whereas if you produce the class files via javac -cp lombok.jar Test.java there's an explicit nullcheck there.

Expected behavior
Let's go with: Generate the null check. One could make an argument that build() will take care of it in the long term, but earlier is probably better here. Also, ecj-produced class files rarely end up in production so keeping the null check is far less backwards incompatible.

Version info (please complete the following information):

• Lombok version: 1.18.6

context: https://groups.google.com/forum/?hl=en#!topic/project-lombok/zHMnp1o1zCw