Format postfix expressions: !, ++, and --. (#1352)

This also includes temporary formatting support for a few syntactic
forms that the postfix tests needed to use. I left TODOs in there so
that we know that support for those isn't complete yet.

lib/src/front_end/ast_node_visitor.dart

@@ -1093,7 +1093,16 @@ class AstNodeVisitor extends ThrowingAstVisitor<Piece> with PieceFactory {
 
   @override
   Piece visitIndexExpression(IndexExpression node) {
-    throw UnimplementedError();
+    // TODO(tall): Allow splitting before and/or after the `[` when method
+    // chain formatting is fully implemented. For now, we just output the code
+    // so that tests of other language features that contain index expressions
+    // can run.
+    return buildPiece((b) {
+      b.visit(node.target);
+      b.token(node.leftBracket);
+      b.visit(node.index);
+      b.token(node.rightBracket);
+    });
   }
 
   @override
@@ -1403,12 +1412,22 @@ class AstNodeVisitor extends ThrowingAstVisitor<Piece> with PieceFactory {
 
   @override
   Piece visitPostfixExpression(PostfixExpression node) {
-    throw UnimplementedError();
+    return buildPiece((b) {
+      b.visit(node.operand);
+      b.token(node.operator);
+    });
   }
 
   @override
   Piece visitPrefixedIdentifier(PrefixedIdentifier node) {
-    throw UnimplementedError();
+    // TODO(tall): Allow splitting before the `.` when method chain formatting
+    // is fully implemented. For now, we just output the code so that tests
+    // of other language features that contain prefixed identifiers can run.
+    return buildPiece((b) {
+      b.visit(node.prefix);
+      b.token(node.period);
+      b.visit(node.identifier);
+    });
   }
 
   @override
@@ -1429,7 +1448,14 @@ class AstNodeVisitor extends ThrowingAstVisitor<Piece> with PieceFactory {
 
   @override
   Piece visitPropertyAccess(PropertyAccess node) {
-    throw UnimplementedError();
+    // TODO(tall): Allow splitting before the `.` when method chain formatting
+    // is fully implemented. For now, we just output the code so that tests
+    // of other language features that contain property accesses can run.
+    return buildPiece((b) {
+      b.visit(node.target);
+      b.token(node.operator);
+      b.visit(node.propertyName);
+    });
   }
 
   @override

test/expression/postfix.stmt

@@ -0,0 +1,33 @@
+40 columns                              |
+>>> Postfix increment.
+value  ++  ;
+<<<
+value++;
+>>> Postfix decrement.
+value  --  ;
+<<<
+value--;
+>>> Increment and decrement as subexpressions.
+value  ++  -  other --;
+<<<
+value++ - other--;
+>>> Null-assert.
+obj ! ;
+<<<
+obj!;
+>>> Null-assert after method call.
+obj . method() ! ;
+<<<
+obj.method()!;
+>>> Null-assert after property.
+obj . prop ! ;
+<<<
+obj.prop!;
+>>> Null-assert inside method chain.
+obj ! . getter ! . method ( arg ) ! + 3;
+<<<
+obj!.getter!.method(arg)! + 3;
+>>> Null-assert before index and call operators.
+obj ! [ index ] ! ( call ) ! + 3;
+<<<
+obj![index]!(call)! + 3;