Enabling StrokeJoin for paths and rectangles. (#10742)

* Prefix and Suffix support for TextFields

* Adding Tests

* Removing spurious newline.

* Fixing a small problem with the test

* Code review changes

* Code Review Changes

* Review Changes

* Export the new StrokeJoin enum

* Added example for line styles, and enabled line join styles.

* Reverting inadvertent change to main.dart.

* Updated due to code review of engine code

* Removed example.

packages/flutter/lib/src/painting/basic_types.dart

@@ -25,6 +25,7 @@ export 'dart:ui' show
   Shader,
   Size,
   StrokeCap,
+  StrokeJoin,
   TextAlign,
   TextBaseline,
   TextDecoration,