after page load postMessage does not work

[DartBot]

This issue was originally filed by dvar...@skawa.hu

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What steps will reproduce the problem?

1. assign: WindowBase win = (iframe as WindowsBase).contentWindow
2. load: iframe.src = "http://example.com";
3. talk: win.postMessage("Hey!", "*");

What is the expected output? What do you see instead?
The postMessage message arrives to the embedded frame (iframe).
Instead of that, the iframe's message event handler never gets fired.

What version of the product are you using? On what operating system?
1.4.2

Please provide any additional information below.

Here is a complete test case that details what the problem is with the actual output compared to the same dart code compiled to JS.
https://github.com/daniel-v/dart-iframe-test

[sgjesse]

Added Area-Library, Library-Html, Triaged labels.

[alan-knight]

May be related to 20146

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Removed Priority-Unassigned label.
Added Priority-Medium, Accepted labels.