[dart2wasm] Return JS string in StackTrace.current

Avoid copying JS string to Dart.

The test co19/LibTest/convert/Latin1Codec_A01_t01 throws a lot of
exceptions. Before this patch it runs in d8 in 4.1s, after it runs in
1.5s.

Change-Id: I1b9bf06876d63e92c93a03cdbf14587e369978ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/338400
Commit-Queue: Ömer Ağacan <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

Author: osa1

sdk/lib/_internal/wasm/lib/stack_trace_patch.dart

@@ -16,14 +16,14 @@ class StackTrace {
     // Note: We remove the first two frames to prevent including
     // `getCurrentStackTrace` and `StackTrace.current`. On Chrome, the first
     // line is not a frame but a line with just "Error", which we also remove.
-    return _StringStackTrace(JS<String>(r"""() => {
+    return _StringStackTrace(JSStringImpl(JS<WasmExternRef?>(r"""() => {
           let stackString = new Error().stack.toString();
           let frames = stackString.split('\n');
           let drop = 2;
           if (frames[0] === 'Error') {
               drop += 1;
           }
-          return stringToDartString(frames.slice(drop).join('\n'));
-        }"""));
+          return frames.slice(drop).join('\n');
+        }""")));
   }
 }