[Dart] Update architecture and template-syntax now that Dart intl pkg supports currency symbols

[chalin]

Pub package intl 0.13 (see changelog) has added support for currency symbols (https://github.com/dart-lang/intl/issues/59). Update the following Dart example sources (which did match their TS counterparts because of https://github.com/dart-lang/intl/issues/59):

./architecture/dart/lib/sales_tax_component.dart:14:       {{ getTax(amountBox.value) | currency:'USD':false:'1.2-2' }}
./architecture/dart/lib/sales_tax_component.dart:15:      <!-- would like to write currency:'USD':true:'1.2-2' but
./template-syntax/dart/lib/app_component.html:731:  <!-- pipe price to USD and display the $ symbol -->
./template-syntax/dart/lib/app_component.html:732:  <label>Price: </label>{{product['price'] | currency:'USD':false}}

• architecture updates - example(architecture/dart): drop currency pipe workaround #2804
• template syntax updates - docs(template-syntax/dart): updates to match TS #2751