when creating new file editor does not recognize 'part of' statement

[DartBot]

This issue was originally filed by @unicomp21

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What steps will reproduce the problem?

1. Take an existing library
2. Add a new file, cell_noise.dart, and put "part of rad_textures;" at the top
3. Update "rad_textures.dart" with "part 'src/cell_noise.dart';"
4. The editor does not recognize the new part unless it is closed and reopened.

What is the expected output? What do you see instead?
editor should recognize new part. editor shows errors for the new part.

What version of the product are you using? On what operating system?
latest, linux

Please provide any additional information below.

[iposva-google]

Added Area-Editor, Triaged labels.

[clayberg]

Set owner to @scheglov.
Added this to the M4 milestone.

[scheglov]

Model or analysis server problem.

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Set owner to @danrubel.

[danrubel]

Removed this from the M4 milestone.
Added this to the M5 milestone.
Added Duplicate label.
Marked as being merged into #8147.