Spec parse null aware elements

Change-Id: Ib61fea3bec110c0a2c560c965912c51a1c4e22cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/368569
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>

tools/spec_parser/Dart.g

@@ -4,6 +4,8 @@
 
 // CHANGES:
 //
+// v0.44 Support null-aware elements.
+//
 // v0.43 Change rule structure such that the association of metadata
 // with non-terminals can be explained in a simple and consistent way.
 // The derivable terms do not change. Remove `metadata` from the kind
@@ -732,13 +734,24 @@ elements
     ;
 
 element
-    :    expressionElement
+    :    nullAwareExpressionElement
+    |    nullAwareMapElement
+    |    expressionElement
     |    mapElement
     |    spreadElement
     |    ifElement
     |    forElement
     ;
 
+nullAwareExpressionElement
+    :    '?' expression
+    ;
+
+nullAwareMapElement
+    :    '?' expression ':' '?'? expression
+    |    expression ':' '?' expression
+    ;
+
 expressionElement
     :    expression
     ;

tools/spec_parser/dart_spec_parser/Dart.g4

@@ -10,6 +10,14 @@
 // of `forLoopParts` where the iteration variable is an existing variable
 // in scope (this is not implemented, is inconsistent anyway).
 //
+// v0.45 Support null-aware elements.
+//
+// v0.44 Change rule structure such that the association of metadata
+// with non-terminals can be explained in a simple and consistent way.
+// The derivable terms do not change. Remove `metadata` from the kind
+// of `forLoopParts` where the iteration variable is an existing variable
+// in scope (this is not implemented, is inconsistent anyway).
+//
 // v0.43 Support updated augmented `extensionDeclaration`.
 //
 // v0.42 Add missing `enumEntry` update for augmentations.
@@ -738,13 +746,24 @@ elements
     ;
 
 element
-    :    expressionElement
+    :    nullAwareExpressionElement
+    |    nullAwareMapElement
+    |    expressionElement
     |    mapElement
     |    spreadElement
     |    ifElement
     |    forElement
     ;
 
+nullAwareExpressionElement
+    :    '?' expression
+    ;
+
+nullAwareMapElement
+    :    '?' expression ':' '?'? expression
+    |    expression ':' '?' expression
+    ;
+
 expressionElement
     :    expression
     ;