[a11y] web does not create correct dialog role for dialog #153791
Description
Steps to reproduce
Run a11y_assessment app with chrome
https://github.com/flutter/flutter/tree/master/dev/a11y_assessments
- click dialog
- click show dialog
See the semantics dump contains a scopesRoute
Details
``` SemanticsNode#0 │ Rect.fromLTRB(0.0, 0.0, 1200.0, 549.0) │ └─SemanticsNode#1 │ Rect.fromLTRB(0.0, 0.0, 1200.0, 549.0) │ textDirection: ltr │ ├─SemanticsNode#55 │ │ Rect.fromLTRB(0.0, 0.0, 1200.0, 549.0) │ │ sortKey: OrdinalSortKey#dfbe2(order: 0.0) │ │ │ └─SemanticsNode#56 │ │ Rect.fromLTRB(0.0, 0.0, 1200.0, 549.0) │ │ flags: scopesRoute │ │ │ └─SemanticsNode#57 │ │ Rect.fromLTRB(40.0, 233.0, 1160.0, 316.0) │ │ label: "This is a typical dialog." │ │ textDirection: ltr │ │ elevation: 6.0 │ │ │ ├─SemanticsNode#58 │ │ Rect.fromLTRB(8.0, 43.0, 64.0, 75.0) │ │ actions: focus, tap │ │ flags: isButton, hasEnabledState, isEnabled, isFocusable │ │ label: "OK" │ │ textDirection: ltr │ │ │ └─SemanticsNode#59 │ Rect.fromLTRB(64.0, 43.0, 131.2, 75.0) │ actions: focus, tap │ flags: isButton, hasEnabledState, isEnabled, isFocusable │ label: "Cancel" │ textDirection: ltr │ └─SemanticsNode#54 Rect.fromLTRB(0.0, 0.0, 1200.0, 549.0) actions: dismiss, tap label: "Dismiss" textDirection: ltr sortKey: OrdinalSortKey#f9f86(order: 1.0) ```but the dom tree doesn't create a semantics node for role=dialog
This is causing the screen reader to not recognize the route change
Actual results
It should generate one semantics node with role=dialog.
Logs
Logs
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Flutter Doctor output
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