dart2js: Spurious error on switch statement with null case.

[leafpetersen]

The code below is valid NNBD code, and is accepted by the VM, DDC, and the analyzer, but is rejected by dart2js:

leafp-macbookpro:analyzer leafp$ ~/src/dart-repo/sdk/xcodebuild/ReleaseX64/dart-sdk/bin/dartanalyzer --enable-experiment=non-nullable ~/tmp/test.dart
Analyzing /Users/leafp/tmp/test.dart...
  hint • The return type of 'main' cannot be inferred. • /Users/leafp/tmp/test.dart:4:1 • inference_failure_on_function_return_type
1 hint found.
leafp-macbookpro:analyzer leafp$ ~/src/dart-repo/sdk/xcodebuild/ReleaseX64/dart-sdk/bin/dartdevc --enable-experiment=non-nullable -o out.js ~/tmp/test.dart
leafp-macbookpro:analyzer leafp$ ~/src/dart-repo/sdk/xcodebuild/ReleaseX64/dart-sdk/bin/dart --enable-experiment=non-nullable ~/tmp/test.dart
leafp-macbookpro:analyzer leafp$ ~/src/dart-repo/sdk/xcodebuild/ReleaseX64/dart-sdk/bin/dart2js --enable-experiment=non-nullable ~/tmp/test.dart
../../../../../tmp/test.dart:6:3:
Error: 'case' expressions do not all have type 'E'.
  switch (e) {
  ^
../../../../../tmp/test.dart:7:12:
Info: 'case' expression of type 'E'.
    case E.A:
           ^
../../../../../tmp/test.dart:11:10:
Info: 'case' expression of type 'Null'.
    case null:
         ^
Error: Compilation failed.

enum E {
  A,
  B}
main() {
  E? e = null;
  switch (e) {
    case E.A:
    return;
    case E.B:
    return;
    case null:
    return;
  }
}

cc @yjbanov

[rakudrama]

The code that implements this check contains

    // TODO(32557): Remove this when issue 32557 is fixed.

I am seeing if #32557 is fixed.

[sigmundch]

@rakudrama - should we basically delete all the checks we have?

If not, seems this was triggering the "inconsistent types" rule, which we could fix by considering all types to be nullable by default and allowing the Null type.