Don't write a trailing comma in extension type representation clauses. (#1518)

I guess trailing commas aren't allowed there. Who knew?

Fix #1505.

Author: munificent

lib/src/front_end/ast_node_visitor.dart

@@ -1574,7 +1574,8 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
   void visitRepresentationDeclaration(RepresentationDeclaration node) {
     pieces.visit(node.constructorName);
 
-    var builder = DelimitedListBuilder(this);
+    var builder =
+        DelimitedListBuilder(this, const ListStyle(commas: Commas.nonTrailing));
     builder.leftBracket(node.leftParenthesis);
     builder.add(pieces.build(() {
       writeParameter(

test/tall/declaration/extension_type.unit

@@ -12,7 +12,7 @@ extension type const A.name(int a) {}
 extension type A<T extends int, R>(int a) {}
 <<<
 extension type A<T extends int, R>(
-  int a,
+  int a
 ) {}
 >>> Indentation in body.
 extension type A(int a) {
@@ -39,15 +39,15 @@ extension type E(T i) implements I, J {}
 extension type LongExtensionType(LongTypeName a) {}
 <<<
 extension type LongExtensionType(
-  LongTypeName a,
+  LongTypeName a
 ) {}
 >>> Split in representation with implements clause.
 extension type LongExtensionType(LongTypeName a) implements Something {
   method() {;}
 }
 <<<
 extension type LongExtensionType(
-  LongTypeName a,
+  LongTypeName a
 )
     implements Something {
   method() {

test/tall/declaration/extension_type_comment.unit

@@ -8,7 +8,7 @@ implements /*k*/ I1 /*l*/ , /*m*/ I2 /*n*/ { /*o*/ } /*p*/
 extension /*b*/ type /*c*/ A
 /*d*/ ( /*e*/
   @ /*f*/ override /*g*/
-  int /*h*/ a /*i*/,
+  int /*h*/ a /*i*/
 ) /*j*/
     implements /*k*/
         I1 /*l*/, /*m*/
@@ -54,7 +54,7 @@ name // j
   @ // l
   required // m
   int // n
-  a, // o
+  a // o
 ) // p
     implements // q
         I // r

test/tall/regression/1500/1505.unit

@@ -0,0 +1,9 @@
+>>>
+extension type JSExportedDartFunction._(
+  JSExportedDartFunctionRepType _jsExportedDartFunction
+) implements JSFunction {}
+<<<
+extension type JSExportedDartFunction._(
+  JSExportedDartFunctionRepType _jsExportedDartFunction
+)
+    implements JSFunction {}