Get the test updater script working again.

When I made the style changes language versions for Dart 3.8, I changed
how tests are written to allow a single test to have different expected
outputs at different language versions.

I was pressed for time, so that broke the automated test updater which
doesn't know how to handle versions. This gets it working again.

In the process, I decided to tweak how the final newline in test files
is handled. For years, I avoided having a trailing newline because I
though it would confuse the formatter. At some point that may have even
been true. But it turns out to be fine and it's more idiomatic to have a
trailing newline at the end of a text file.

The test updater now ensures that and I went ahead and normalized all
of the tests to have trailing newlines. There are no other changes to
the test files.

Author: munificent

test/short/comments/cascades.stmt

@@ -64,4 +64,4 @@ receiver
   ..cascade()
 
   // comment
-  ..cascade();
+  ..cascade();

test/short/comments/classes.unit

@@ -195,4 +195,4 @@ class Foo< /* comment */ T> {}
 class Foo< /* comment */TypeParameter> {}
 <<<
 class Foo< /* comment */
-    TypeParameter> {}
+    TypeParameter> {}

test/short/comments/enums.unit

@@ -116,4 +116,4 @@ enum Foo {
   a(// c
       ),
   b()
-}
+}

test/short/comments/expressions.stmt

@@ -114,4 +114,4 @@ y) = o;
 y) = o;
 <<<
 (x && // c
-    y) = o;
+    y) = o;

test/short/comments/extensions.unit

@@ -193,4 +193,4 @@ extension A on B {}
 
 /**
 */
-extension A on B {}
+extension A on B {}

test/short/comments/functions.unit

@@ -247,4 +247,4 @@ function(
   @Other
   @Third
   longParameter2,
-) {}
+) {}

test/short/comments/generic_methods.unit

@@ -50,4 +50,4 @@ void foo/*<S>*/(var /*=S*/ x) {
 <<<
 void foo/*<S>*/(var/*=S*/ x) {
   var/*=S*/ y = x;
-}
+}

test/short/comments/lists.stmt

@@ -110,4 +110,4 @@ var list = [
 <<<
 var list = [
   // comment
-];
+];

test/short/comments/maps.stmt

@@ -115,4 +115,4 @@ var map = {
 <<<
 var map = {
   // comment
-};
+};

test/short/comments/mixed.unit

@@ -103,4 +103,4 @@ class C {
 //// not a doc comment
 //commentedOut() {}
   method() {}
-}
+}