[dart2wasm] Make WasmTable a _WasmBase subclass
[dart2wasm] Make WasmTable a _WasmBase subclass

This fixes `isWasmType` checks for `WasmTable`. Currently this doesn't
cause any test failures, but dynamic invocations incorrectly consider
`WasmTable` as a potential receiver, which causes these warnings:

    Unimplemented external member WasmTable.[] at file:///...

Change-Id: Icc2b30e21e9d5c1de3229f6d55e3a6dc962b648b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/271522
Reviewed-by: Aske Simon Christensen <589eb3fd9a84f06d0ac46d3aa71dddcfb0814ccd@google.com>
Commit-Queue: Ömer Ağacan <d15e39adaef0c3a64e4ea5d68f5c1b5be1f04e29@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