[dart2wasm] Make string globals non-nullable

Those globals should never be able to old a `null` value.

Strictly speaking the `js-string-builtin` spec also says
```
If an import refers to the imported string namespace, then the import
type is matched against an extern type of (global (ref extern))
```

And `ref extern` is non-nullable (compared with `externref` which
is shorthand for `ref null extern`)


Change-Id: I0e2638fbfeb6d59f740320c7d4167fcff3d793e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/457980
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Ömer Ağacan <[email protected]>

Author: mkustermann

pkg/dart2wasm/lib/translator.dart

@@ -1860,13 +1860,13 @@ class Translator with KernelNodes {
       // runtime.
       final i = internalizedStringsForJSRuntime.length;
       internalizedString = module.globals.import('s', '$i',
-          w.GlobalType(w.RefType.extern(nullable: true), mutable: false));
+          w.GlobalType(w.RefType.extern(nullable: false), mutable: false));
       internalizedStringsForJSRuntime.add(s);
     } else {
       internalizedString = module.globals.import(
         'S',
         s,
-        w.GlobalType(w.RefType.extern(nullable: true), mutable: false),
+        w.GlobalType(w.RefType.extern(nullable: false), mutable: false),
       );
     }
     _internalizedStringGlobals[(module, s)] = internalizedString;

pkg/dart2wasm/test/ir_tests/deferred.constant.wat

@@ -25,8 +25,8 @@
     (field $fun (ref $#Closure-0-1)))))
   (type $type256 <...>)
   (type $#DummyStruct <...>)
-  (global $S.globalH1Bar< (import "S" "globalH1Bar<") externref)
-  (global $S.globalH0Foo (import "S" "globalH0Foo") externref)
+  (global $S.globalH1Bar< (import "S" "globalH1Bar<") (ref extern))
+  (global $S.globalH0Foo (import "S" "globalH0Foo") (ref extern))
   (global $global29 (ref $#DummyStruct) <...>)
   (global $"C28 _InterfaceType" (ref $_InterfaceType) <...>)
   (global $"C334 \"h0\"" (ref $JSStringImpl) <...>)

pkg/dart2wasm/test/ir_tests/hello.wat

@@ -4,7 +4,7 @@
   (type $JSStringImpl (sub final $#Top (struct
     (field $field0 i32)
     (field $_ref externref))))
-  (global $"S.hello world" (import "S" "hello world") externref)
+  (global $"S.hello world" (import "S" "hello world") (ref extern))
   (global $"C329 \"hello world\"" (ref $JSStringImpl)
     (i32.const 4)
     (global.get $"S.hello world")