Add more tests for type arguments in object patterns and remove a TOD…

…O. (#1401)

* Add more tests for type arguments in object patterns and remove a TODO.

I'm honestly not sure what the TODO was even trying to say. But I think
the formatter behaves like we want here so there's nothing TODO as far
as I can tell. But this corner did seem a little under tested, so added
some more.

* Fix indentation collapsing.

A block indent shouldn't be able to partially eat an expression-sized
collapsible indentation. The only time we want to actually collapse is
when we have two equivalent expression-sized indentations.

lib/src/back_end/code_writer.dart

@@ -153,15 +153,19 @@ class CodeWriter {
     var parentIndent = _indentStack.last.indent;
     var parentCollapse = _indentStack.last.collapsible;
 
-    if (canCollapse) {
-      // Increase the indent and the collapsible indent.
-      _indentStack.add(_Indent(parentIndent + indent, parentCollapse + indent));
-    } else if (parentCollapse > indent) {
-      // All new indent is collapsed with the existing collapsible indent.
-      _indentStack.add(_Indent(parentIndent, parentCollapse - indent));
+    if (parentCollapse == indent) {
+      // We're indenting by the same existing collapsible amount, so collapse
+      // this new indentation with that existing one.
+      _indentStack.add(_Indent(parentIndent, 0));
+    } else if (canCollapse) {
+      // We should never get multiple levels of nested collapsible indentation.
+      assert(parentCollapse == 0);
+
+      // Increase the indentation and note that it can be collapsed with
+      // further indentation.
+      _indentStack.add(_Indent(parentIndent + indent, indent));
     } else {
-      // Use up the collapsible indent (if any) and then indent by the rest.
-      indent -= parentCollapse;
+      // Regular indentation, so just increase the indent.
       _indentStack.add(_Indent(parentIndent + indent, 0));
     }
   }

test/pattern/object.stmt

@@ -165,18 +165,25 @@ if (obj case Foo(
 )) {
   ;
 }
->>> Split in type argument.
+>>> Split in type argument with no field subpatterns.
 if (obj case LongClassName<First, Second>()) {;}
 <<<
-### TODO(tall): It formats like this if there's no elements. Similarly with lists, maps etc.
 if (obj
     case LongClassName<
-        First,
-        Second
+          First,
+          Second
         >()) {
   ;
 }
->>> Split in type argument and body.
+>>> Prefer splitting in field subpatterns instead of type arguments.
+if (obj case Foo<First, Second>(first: 1)) {;}
+<<<
+if (obj case Foo<First, Second>(
+  first: 1,
+)) {
+  ;
+}
+>>> Split in type argument but not field subpatterns.
 if (obj case LongClassName<First, Second, Third>(first: 1, second: 2, third: 3)) {;}
 <<<
 if (obj case LongClassName<
@@ -186,3 +193,18 @@ if (obj case LongClassName<
 >(first: 1, second: 2, third: 3)) {
   ;
 }
+>>> Split in type arguments and field subpatterns.
+if (obj case LongClassName<First, Second, Third>(first: 1, second: 2, third: 3, fourth: 4)) {;}
+<<<
+if (obj case LongClassName<
+  First,
+  Second,
+  Third
+>(
+  first: 1,
+  second: 2,
+  third: 3,
+  fourth: 4,
+)) {
+  ;
+}