Use setRange when copying output chunks to the final buffer in `Cod…

…edBufferWriter` (#887)

Similar to #885, this optimizes some more buffer copying to `memcpy`.

Results from the same benchmark in #885:

|                              | Before     | After      | Diff                |
|------------------------------|------------|------------|---------------------|
| AOT                          | 114,713 us | 109,838 us | - 4,875 us, -4.2%   |
| JIT                          |  91,960 us |  92,887 us | +   927 us, +1.0%   |
| dart2js -O4                  | 259,125 us | 257,000 us | - 2,125 us, -0.8%   |
| dart2wasm --omit-type-checks | 196,909 us | 182,333 us | -14,576 us, -7.4%   |

AOT and JIT tested on x64.

protobuf/lib/src/protobuf/coded_buffer_writer.dart

@@ -141,21 +141,21 @@ class CodedBufferWriter {
           }
           buffer[outPos++] = v;
         } else {
-          // action is an amount of bytes to copy from _outputChunks into the
-          // buffer.
+          // `action` is an amount of bytes to copy from `_outputChunks` into
+          // the buffer.
           var bytesToCopy = action;
           while (bytesToCopy > 0) {
             final Uint8List chunk = _outputChunks[chunkIndex];
             final int bytesInChunk = _outputChunks[chunkIndex + 1];
 
-            // Copy at most bytesToCopy bytes from the current chunk.
+            // Copy at most `bytesToCopy` bytes from the current chunk.
             final leftInChunk = bytesInChunk - chunkPos;
             final bytesToCopyFromChunk =
                 leftInChunk > bytesToCopy ? bytesToCopy : leftInChunk;
-            final endPos = chunkPos + bytesToCopyFromChunk;
-            while (chunkPos < endPos) {
-              buffer[outPos++] = chunk[chunkPos++];
-            }
+            buffer.setRange(
+                outPos, outPos + bytesToCopyFromChunk, chunk, chunkPos);
+            chunkPos += bytesToCopyFromChunk;
+            outPos += bytesToCopyFromChunk;
             bytesToCopy -= bytesToCopyFromChunk;
 
             // Move to the next chunk if the current one is exhausted.