matInput placeholder position with prefix doesn't align with the input field

[dhuallanca]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

placeholder should be aligned with the input field

What is the current behavior?

placeholder is displayed after the prefix

What are the steps to reproduce?

https://stackblitz.com/edit/angular-jgpuyq?file=app/form-field-prefix-suffix-example.html

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

UI design

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

@angular/material: "^5.1.0",
@angular/core: "^5.2.1",

Chrome

Is there anything else we should know?

[dhuallanca]

It should work like material design guidelines:

https://material.io/guidelines/components/text-fields.html#text-fields-field-types

[mmalerba]

this should be done via a text mask in the future, #7645 is tracking the issue

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