Implement lineTerminator in MemoryStdout Fake (#143608)

https://dart-review.googlesource.com/c/sdk/+/326761/24/sdk/lib/io/stdio.dart#380
added a `lineTerminator` field to `Stdout`.

Add that field to the fake in packages/test.

Fixes flutter/flutter#143614

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packages/flutter_tools/test/src/fakes.dart

@@ -193,6 +193,13 @@ class MemoryStdout extends MemoryIOSink implements io.Stdout {
   }
   bool _hasTerminal = true;
 
+  @override
+  // ignore: override_on_non_overriding_member
+  String get lineTerminator => '\n';
+  set lineTerminator(String value) {
+    throw UnimplementedError('Setting the line terminator is not supported');
+  }
+
   @override
   io.IOSink get nonBlocking => this;