Align doubles to 8 bytes in the StandardMessageCodec (#10758)

See https://github.com/flutter/flutter/issues/10701

bin/internal/engine.version

-f741647d4977410ff8e2c02160eaa02bcb1f0a2b
+784e9756720f7f6daa15c95ba3df6215bb54783f

packages/flutter/lib/src/foundation/serialization.dart

@@ -55,6 +55,7 @@ class WriteBuffer {
 
   /// Write an Float64 into the buffer.
   void putFloat64(double value) {
+    _alignTo(8);
     _eightBytes.setFloat64(0, value, Endianness.HOST_ENDIAN);
     _buffer.addAll(_eightBytesAsList);
   }
@@ -150,6 +151,7 @@ class ReadBuffer {
 
   /// Reads a Float64 from the buffer.
   double getFloat64() {
+    _alignTo(8);
     final double value = data.getFloat64(_position, Endianness.HOST_ENDIAN);
     _position += 8;
     return value;

packages/flutter/test/services/message_codecs_test.dart

@@ -177,6 +177,14 @@ void main() {
       ];
       _checkEncodeDecode<dynamic>(standard, message);
     });
+    test('should align doubles to 8 bytes', () {
+      _checkEncoding<dynamic>(
+        standard,
+        1.0,
+        <int>[6, 0, 0, 0, 0, 0, 0, 0,
+              0, 0, 0, 0, 0, 0, 0xf0, 0x3f],
+      );
+    });
   });
 }