[dart2wasm] Remove assertion that named param Symbol cannot be private.

Though named parameters cannot be private (this is checked by the
CFE/analyzer) it is not a static error to specify a named argument
beginning with "_". Instead this should be a runtime error (resulting in
an NSM since the caller signature won't match the target callee's).

We could move this assert to the call sites but there are several that
would need to have this check. And the CFE should be validating this
anyway so it should be safe/simplest to just remove this assert.

Fixes: https://github.com/dart-lang/sdk/issues/63201
Change-Id: I511b56c7a3b0964ef3db744a75b574dd9551e6ab
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/498340
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

pkg/dart2wasm/lib/symbols.dart

@@ -23,8 +23,6 @@ class Symbols {
   }
 
   SymbolConstant symbolForNamedParameter(String name) {
-    // Named parameters cannot be private.
-    assert(!name.startsWith('_'));
     return SymbolConstant(name, null);
   }