Incorrect Completion Value for ClassDeclaration statement

[JosephPecoraro]

Completion value should be empty for ClassDeclaration statements. Spec

Test:

eval(`1; class C {}`)

Expected: 1
Actual: C

[dilijev]

Repro'd. Here's the behavior of various engines on this case:

## Source
print(eval(`1; class C {}`))

┌──────────────┬───────────────┐
│ node         │ [Function: C] │
│ node-ch      │               │
│ node-nightly │               │
├──────────────┼───────────────┤
│ ch-master    │ class C {}    │
│ ch-2.0       │               │
│ ch-1.4.2     │               │
├──────────────┼───────────────┤
│ sm           │ 1             │
│ d8           │               │
└──────────────┴───────────────┘

[dilijev]

In case it helps diagnose/fix: we correctly have an empty completion if the class is assigned to a binding (var/let/const):

## Source
print(eval(`1; let C = class C {}`))

┌──────────────┬───┐
│ d8           │ 1 │
│ sm           │   │
│ ch-master    │   │
│ ch-2.0       │   │
│ ch-1.4.2     │   │
│ node-ch      │   │
│ node-nightly │   │
│ node         │   │
└──────────────┴───┘

Edit: we also do the right thing for FunctionDeclaration

[JosephPecoraro]

Yes, VariableDeclaration, FunctionDeclaration, and ClassDeclaration should all have a completion value of undefined.

[EdMaurer]

Non-compliant, yes, but is this having any negative effect?

[dilijev]

Not sure if we have a measurement of that. Anyone who relies on the return value of eval is potentially affected. Also potentially affects REPL scenarios like Node-ChakraCore's REPL and F12 Console REPL which display the result of an expression or statement which might be a completion value, but those situations might have special rules.