Don't emit dangling comma in object literal.

frog/tests/leg/class_codegen2_test.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+// Test that parameters keep their names in the output.
+
+#import("compiler_helper.dart");
+#import("parser_helper.dart");
+
+final String TEST_ONE = @"""
+class A { foo() => 499; }
+class B { bar() => 42; }
+
+main() {
+  new A().foo();
+  new B().bar();
+}
+""";
+
+final String TEST_TWO = @"""
+class A {
+  foo() => 499;
+  bar() => 42;
+}
+
+main() {
+  new A().foo();
+  new A().bar();
+}
+""";
+
+final String TEST_THREE = @"""
+class A {
+  foo() => 499;
+  bar() => 42;
+}
+
+class B extends A {
+  foo() => -499;
+  bar() => -42;
+}
+
+var y;
+foo(i) {
+  if (0 != i) {
+    y--;
+    foo(i - 1);
+    y++;
+  }
+}
+
+makeStaticInliningHard() {
+  y = 0;
+  foo(10);
+  return 0 == y;
+}
+
+
+// id returns [x] in a way that should be difficult to predict statically.
+id(x) {
+  y = x;
+  foo(10);
+  return y;
+}
+
+main() {
+  var a = new A();
+  var b = new B();
+  var x = a;
+  if (makeStaticInliningHard()) x = b;
+  x.foo();
+  x.bar();
+}
+""";
+
+main() {
+  // At some point Dart2js generated bad object literals with dangling commas:
+  // { a: true, }. Make sure this doesn't happen again.
+  RegExp danglingComma = const RegExp(@',[ \n]*}');
+  String generated = compileAll(TEST_ONE);
+  Expect.isFalse(danglingComma.hasMatch(generated));
+
+  generated = compileAll(TEST_TWO);
+  Expect.isFalse(danglingComma.hasMatch(generated));
+
+  generated = compileAll(TEST_THREE);
+  Expect.isFalse(danglingComma.hasMatch(generated));
+}