[patch][cla] optimizeCss can produce invalid CSS for unresolved @imports

[jameyg]

When expanding/flattening an @import, optimizeCss may generate invalid CSS (​http://www.w3.org/TR/CSS2/syndata.html#at-rules) if the flattened css contains an @import that cannot be resolved.

for example:

[app.css]
@import a.css
@import b.css

[a.css]
h1{font-size:110%;}

[b.css]
@import http://cdn.something.com/res/ext.css

app.css would then get improperly flattened to (@import is incorrectly placed):

h1{font-size:110%;}
@import http://cdn.something.com/res/ext.css

[bill]

This was fixed in ​27a481239d94044b93053a82bfd82518facb5e8d.