[records] Allow "()" as an empty record expression. (#2535)

* [records] Allow "()" as an empty record expression.

* Update version.

Author: munificent

accepted/future-releases/records/records-feature-specification.md

@@ -4,7 +4,7 @@ Author: Bob Nystrom
 
 Status: Accepted
 
-Version 1.12 (see [CHANGELOG](#CHANGELOG) at end)
+Version 1.13 (see [CHANGELOG](#CHANGELOG) at end)
 
 ## Motivation
 
@@ -122,6 +122,8 @@ not captured by the grammar. It is a compile-time error if a record has any of:
 
 *   Only one positional field and no trailing comma.
 
+*   No fields and a trailing comma. *The expression `(,)` isn't allowed.*
+
 *   A field named `hashCode`, `runtimeType`, `noSuchMethod`, or `toString`.
 
 *   A field name that starts with an underscore.
@@ -138,8 +140,7 @@ var number = (1);  // The number 1.
 var record = (1,); // A record containing the number 1.
 ```
 
-There is no syntax for a zero-field record expression. Instead, there is a
-static constant `empty` on `Record` that returns the empty record.
+The expression `()` refers to the constant empty record with no fields.
 
 ### Record type annotations
 
@@ -347,8 +348,8 @@ fields are) and collection literals.
 ### Constants
 
 Record expressions can be constant and potentially constant expressions. A
-record expression is a compile-time constant expression if and only if all its
-field expressions are compile-time constant expressions.
+record expression is a compile-time constant expression if and only if all of
+its field expressions are compile-time constant expressions.
 
 *This is true whether the expression occurs in a constant context or not, which
 means that a record expression can be used directly as a parameter default value
@@ -587,6 +588,10 @@ covariant in their field types.
 
 ## CHANGELOG
 
+### 1.13
+
+- Introduce `()` syntax for empty record expressions and remove `Record.empty`.
+
 ### 1.12
 
 - Include record types in `typeNotVoid`. This allows them to appear in `is` and