Make an empty identifier a scanning error.

tests/language/string_interpolation9_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.
+//
+// Almost valid string interpolation syntax.
+
+main() {
+  var x;
+
+  x = "$";    /// 1: compile-time error
+  x = "x$";   /// 2: compile-time error
+  x = "$x$";  /// 3: compile-time error
+  x = "$$x";  /// 4: compile-time error
+  x = "$ ";   /// 5: compile-time error
+
+  x = '$';    /// 6: compile-time error
+  x = 'x$';   /// 7: compile-time error
+  x = '$x$';  /// 8: compile-time error
+  x = '$$x';  /// 9: compile-time error
+  x = '$ ';   /// 10: compile-time error
+
+  x = """$""";    /// 11: compile-time error
+  x = """x$""";   /// 12: compile-time error
+  x = """$x$""";  /// 13: compile-time error
+  x = """$$x""";  /// 14: compile-time error
+  x = """$ """;   /// 15: compile-time error
+
+  x = '''$''';    /// 16: compile-time error
+  x = '''x$''';   /// 17: compile-time error
+  x = '''$x$''';  /// 18: compile-time error
+  x = '''$$x''';  /// 19: compile-time error
+  x = '''$ ''';   /// 20: compile-time error
+
+  return x;
+}