fix(compiler): Implement Token#toString for Operator

Include a case for `TokenType.Operator`. Closes angular#4049

Author: Tim Blasi

modules/angular2/src/core/change_detection/parser/lexer.ts

@@ -75,9 +75,10 @@ export class Token {
  toString(): string {
  switch (this.type) {
  case TokenType.Character:
- case TokenType.String:
  case TokenType.Identifier:
  case TokenType.Keyword:
+ case TokenType.Operator:
+ case TokenType.String:
  return this.strValue;
  case TokenType.Number:
  return this.numValue.toString();