Fix assert failure in await expression with non-null assertion

[Copilot]

This PR fixes a debug assertion failure that occurs when using NODE_ENV=development with await expressions followed by non-null assertions.

Issue

The following code would cause a debug assertion failure when compiled with NODE_ENV=development:

async function test() {
    const test = (await null as any)!;
}

The error was:

Error: Debug Failure. False expression: Unexpected node.
Verbose Debug Information: Node PartiallyEmittedExpression did not pass test 'isLeftHandSideExpression'.

Root Cause

In src/compiler/transformers/ts.ts, the visitNonNullExpression function was incorrectly using isLeftHandSideExpression as the test when visiting the inner expression:

function visitNonNullExpression(node: NonNullExpression): Expression {
    const expression = visitNode(node.expression, visitor, isLeftHandSideExpression); // ❌ Wrong test
    Debug.assert(expression);
    return factory.createPartiallyEmittedExpression(expression, node);
}

The issue is that await expressions are unary expressions, not left-hand side expressions, so the debug assertion would fail when NODE_ENV=development enabled debug checks.

Solution

Changed the visitor test from isLeftHandSideExpression to isExpression to match the pattern used by similar transformation functions like visitAssertionExpression and visitSatisfiesExpression:

function visitNonNullExpression(node: NonNullExpression): Expression {
    const expression = visitNode(node.expression, visitor, isExpression); // ✅ Correct test
    Debug.assert(expression);
    return factory.createPartiallyEmittedExpression(expression, node);
}

Testing

• Added comprehensive test case awaitNonNullAssertion.ts covering various await + non-null assertion patterns
• Verified 5268+ related tests pass including all await, assertion, and expression transformation tests
• Confirmed the original failing code now compiles without assertion errors

Fixes #23.

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