select(multiple): checkboxes do not appear when using ng-multiple="true"

[Splaktar]

Bug

CodePen and steps to reproduce the issue:

CodePen Demo which demonstrates the issue:

https://codepen.io/Splaktar/pen/YoOEMm?editors=1010#0

Detailed Reproduction Steps:

1. Open the select menu
2. Note that there are no checkboxes, but you can select multiple options

What is the expected behavior?

Checkboxes appear for multiple select options.

What is the current behavior?

PR #7455 added checkboxes for options in a multiple select, but it did not account for the ng-multiple case which is undocumented and has no tests.

What is the use-case or motivation for changing an existing behavior?

Consistency. If you use multiple you get checkboxes, if you use ng-multiple="true" or some other truthy expression, you get no checkboxes.

Which versions of AngularJS, Material, OS, and browsers are affected?

• AngularJS: 1.5.5+
• AngularJS Material: 1.1.0-1.1.19
• OS: macOS
• Browsers: Chrome 75

Is there anything else we should know? Stack Traces, Screenshots, etc.

multiple:

ng-multiple="true":

I ran into this inconsistency while working on a11y issues: #10967 and #10748.

[marosoft]

@Splaktar isn't it related to #9705?

[Splaktar]

@marosoft yes, I forgot about that one, thank you! I'll close this one out as that issue lays out why ng-multiple support is not documented or tested (it was removed from AngularJS in 1.2.0). It makes sense to leave that one planned for 1.2.0 of AngularJS Material since it's a breaking change.