[dart2wasm] Handle switches with just a default case
[dart2wasm] Handle switches with just a default case

Currently switch compiler looks at literals in alternatives to determine
how to check for equality.

When a switch statement only has a default case (no literals) the code
fails to determine the type of the expression in case statement and
assumes `bool`. Fixed by checking for these cases.

These tests now pass:

- co19/Language/Statements/Switch/syntax_t01
- co19/Language/Statements/Switch/execution_t02
- language/nnbd/flow_analysis/write_promoted_value_in_switch_test

Change-Id: If277b5a2dd04dd84d8d4ab6227d95c049d71f0e5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/264681
Commit-Queue: Ömer Ağacan <d15e39adaef0c3a64e4ea5d68f5c1b5be1f04e29@google.com>
Reviewed-by: Joshua Litt <9f8daed1ab51574c9a48ffacb03e4a1adb4f880c@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk