[macOS] Fix: Memory sanitizer violated when encoding indirect strings (flutter/engine#49995)

Fixes https://github.com/flutter/flutter/issues/142101

@cbracken However, this unit test requires the unit tests to be compiled
with `--asan` to work. Can we add this flag to CI?

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engine/src/flutter/shell/platform/darwin/common/framework/Source/FlutterStandardCodecHelper.cc

@@ -221,7 +221,7 @@ void FlutterStandardCodecHelperWriteUTF8(CFMutableDataRef data,
     // UTF16 length times 3 will fit all UTF8.
     CFIndex buffer_length = length * 3;
     std::vector<UInt8> buffer;
-    buffer.reserve(buffer_length);
+    buffer.resize(buffer_length);
     CFStringGetBytes(value, CFRangeMake(0, length), kCFStringEncodingUTF8, 0,
                      false, buffer.data(), buffer_length, &used_length);
     FlutterStandardCodecHelperWriteSize(data, used_length);

engine/src/flutter/shell/platform/darwin/common/framework/Source/flutter_standard_codec_unittest.mm

@@ -4,6 +4,8 @@
 
 #import "flutter/shell/platform/darwin/common/framework/Headers/FlutterCodecs.h"
 
+#include <CoreFoundation/CoreFoundation.h>
+
 #include "gtest/gtest.h"
 
 FLUTTER_ASSERT_ARC
@@ -215,6 +217,19 @@ TEST(FlutterStandardCodec, CanEncodeAndDecodeStringWithNonBMPCodePoint) {
   CheckEncodeDecode(@"h\U0001F602w", [NSData dataWithBytes:bytes length:8]);
 }
 
+TEST(FlutterStandardCodec, CanEncodeAndDecodeIndirectString) {
+  // This test ensures that an indirect NSString, whose internal string buffer
+  // can't be simply returned by `CFStringGetCStringPtr`, can be encoded without
+  // violating the memory sanitizer. This test only works with `--asan` flag.
+  // See https://github.com/flutter/flutter/issues/142101
+  uint8_t bytes[7] = {0x07, 0x05, 0x68, 0xe2, 0x98, 0xba, 0x77};
+  NSString* target = @"h\u263Aw";
+  // Ensures that this is an indirect string so that this test makes sense.
+  ASSERT_TRUE(CFStringGetCStringPtr((__bridge CFStringRef)target, kCFStringEncodingUTF8) ==
+              nullptr);
+  CheckEncodeDecode(target, [NSData dataWithBytes:bytes length:7]);
+}
+
 TEST(FlutterStandardCodec, CanEncodeAndDecodeArray) {
   NSArray* value = @[ [NSNull null], @"hello", @3.14, @47, @{@42 : @"nested"} ];
   CheckEncodeDecode(value);