parseConciseBody_(params) {
    // The body can be a block or an expression. A '{' is always treated as
    // the beginning of a block.
    if (peek(OPEN_CURLY))
      return this.parseFunctionBody_(params);

    validateParameters(params, this.strictMode_, this.errorReporter_);
    return this.parseAssignmentExpression_(ALLOW_IN);
  }
ConciseBody :
  [lookahead not {] AssignmentExpression
  { FunctionBody }

@param {ParseTree} params
@return {ParseTree}