[next] MessageEvent.source inconsistent between Dartium and dart2js

[DartBot]

This issue was originally filed by felipe...@gmail.com

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What steps will reproduce the problem?

1. Set a MessageEvent listener: window.onMessage.listen((MessageEvent e) { /* Verify source here */ });
2. Open a window: newWindow = window.open("some_page.html", "");
3. newWindow sends a message back to its opener: window.opener.postMessage("Message.", "*");
4. Source verification (newWindow == e.source) returns TRUE on Dart, but FALSE on JavaScript build.

What is the expected output? What do you see instead?
JavaScript build must work as Dart does (expected TRUE on both cases).

What version of the product are you using?
Dart Editor version 1.8.0.dev_04_06 (DEV)
Dart SDK version 1.8.0-dev.4.6

On what operating system?
Linux: Debian Testing (Jessie) - 3.16.0-4-amd64

What browser (if applicable)?
Chrome and Iceweasel (Firefox).

Please provide any additional information below.
No additional information.

[kasperl]

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

[alan-knight]

Review with Terry's upcoming changes.

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Removed Priority-Unassigned label.
Added Priority-Medium, Accepted labels.
Changed the title to: "[next] MessageEvent.source inconsistent between Dartium and dart2js".

[daniel-v]

I run into this with 1.16 as well. Chrome version 50.

Steps for reproductions remain the same.