[dart2wasm] Fix string array copy in UTF8 decoder

Bug introduced with [1] and broke the Flutter engine [2].

[1]: https://dart-review.googlesource.com/c/sdk/+/331187
[2]: flutter/flutter#137120

Change-Id: I334db1bbf1440b53b7a7856f16a0e116d82efae5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/331922
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

sdk/lib/_internal/wasm/lib/convert_patch.dart

@@ -1736,7 +1736,8 @@ class _Utf8Decoder {
       // Pure ASCII.
       assert(size == end - start);
       OneByteString result = OneByteString.withLength(size);
-      oneByteStringArray(result).copy(0, bytes.data, start, size);
+      oneByteStringArray(result)
+          .copy(0, bytes.data, bytes.offsetInBytes + start, size);
       return result;
     }
 

tests/lib/convert/utf8_test.dart

@@ -32,6 +32,16 @@ void testDecodeSlice() {
     Expect.equals("BCD", decoder.convert(ascii, 1, 4));
     Expect.equals("ABCD", decoder.convert(ascii, 0, 4));
 
+    if (ascii is Uint8List) {
+      Expect.equals("ABCDE", decoder.convert(Uint8List.sublistView(ascii, 0)));
+      Expect.equals("ABCDE",
+          decoder.convert(Uint8List.sublistView(ascii, 0, ascii.length)));
+      Expect.equals("CDE", decoder.convert(Uint8List.sublistView(ascii, 2)));
+      Expect.equals("BCD", decoder.convert(Uint8List.sublistView(ascii, 1, 4)));
+      Expect.equals(
+          "ABCD", decoder.convert(Uint8List.sublistView(ascii, 0, 4)));
+    }
+
     Expect.throws(() => decoder.convert(ascii, -1)); //    start < 0.
     Expect.throws(() => decoder.convert(ascii, 6)); //     start > length
     Expect.throws(() => decoder.convert(ascii, 0, -1)); // end < 0
@@ -48,6 +58,18 @@ void testDecodeSlice() {
     Expect.equals("\u0082\u1041", decoder.convert(utf8, 2));
     Expect.equals("\u0081\u0082", decoder.convert(utf8, 0, 4));
     Expect.equals("\u0082", decoder.convert(utf8, 2, 4));
+
+    if (utf8 is Uint8List) {
+      Expect.equals("\u0081\u0082\u1041",
+          decoder.convert(Uint8List.sublistView(utf8, 0)));
+      Expect.equals(
+          "\u0082\u1041", decoder.convert(Uint8List.sublistView(utf8, 2)));
+      Expect.equals(
+          "\u0081\u0082", decoder.convert(Uint8List.sublistView(utf8, 0, 4)));
+      Expect.equals(
+          "\u0082", decoder.convert(Uint8List.sublistView(utf8, 2, 4)));
+    }
+
     Expect.throws(() => decoder.convert(utf8, 1));
     Expect.throws(() => decoder.convert(utf8, 0, 1));
     Expect.throws(() => decoder.convert(utf8, 2, 5));