Crash on invalid abstract export class declaration inside a namespace

[hungryzzz]

Bug description

Hi, I found that asc crashes while compiling the following AssemblyScript code snippet. I would expect a normal compilation error rather than a compiler crash.

Steps to reproduce

// bad.ts
namespace N {
  abstract export class C {}
}

> node ~/assemblyscript/bin/asc --optimizeLevel 2 --shrinkLevel 2 bad.ts -o bad.wasm

Output:

▌ Whoops, the AssemblyScript compiler has crashed during initialize :-(
▌
▌ Here is the stack trace hinting at the problem, perhaps it's useful?
▌
▌ AssertionError: assertion failed
▌     at Y.assert (~/assemblyscript/std/portable/index.js:216:11)
▌     at On.initializeNamespace (~/assemblyscript/src/program.ts:2813:18)
▌     at On.initialize (~/assemblyscript/src/program.ts:1144:18)
▌     at Module.MS (~/assemblyscript/src/index-wasm.ts:354:11)
▌     at Module.De (~/assemblyscript/cli/index.js:714:22)
▌     at async file:///~/assemblyscript/bin/asc.js:33:22

Other findings

1. The valid modifier order inside a namespace does not crash:

namespace N {
  export abstract class C {}
}

2. The same invalid modifier order at the top level reports a normal compilation error instead of crashing:

// good.ts
abstract export class C {}

// ~/warpo/build/warpo/warpo_asc --optimizeLevel 2 --shrinkLevel 2 good.ts -o mutant.wasm

// compilation failed
// ERROR TS2304: Cannot find name 'abstract'.
//    :
//  2 │ abstract export class C{}
//    │ ~~~~~~~~
//    └─ in good.ts(2,1)

// ERROR: compilation failed

AssemblyScript version

b6bda05