support arbitrary expressions in url attributes

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="96" height="96"hspace="10"> Issue by sigmundch
Originally opened as dart-lang/sdk#20286

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works: <img src="foo/{{bar}}">
fails: <img src="foo/{{bar[3]}}">

(just adding a test in import_inliner_test.dart will replicate the issue)

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Jake - since you recently fixed the other issue, I thought maybe you might like to take a look at this one.

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Set owner to @jakemac53.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2049220?v=3" align="left" width="48" height="48"hspace="10"> Comment by sigmundch

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Marked this as blocking #228.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/984921?v=3" align="left" width="48" height="48"hspace="10"> Comment by jakemac53

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Added Started label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/984921?v=3" align="left" width="48" height="48"hspace="10"> Comment by jakemac53

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Fix pending in https://codereview.chromium.org/433913002.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/984921?v=3" align="left" width="48" height="48"hspace="10"> Comment by jakemac53

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Fixed in dart-lang/sdk@4a9130b

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Added Fixed label.