[wesleycho]

• Please check if the PR fulfills these requirements
• The commit message follows our guidelines: https://github.com/angular/universal/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• What modules are related to this pull-request
• express-engine
• grunt-prerender
• gulp-prerender
• hapi-engine
• preboot
• universal-preview
• universal
• webpack-prerender
• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  remove unused proxy imports from some test files
• What is the current behavior? (You can also link to an open issue here)
• What is the new behavior (if this is a feature change)?
• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No
• Other information:
• Add img attributes to ATTRIBUTES constant so setElementProperty can find them

This should fix #331.

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