[ Widget Previews ] Support projects containing libraries with part files (#171027)

Reworked the dependency model to be library based instead of script
based in order to support libraries spanning multiple scripts.

Also refactored test logic to improve maintainability and simply writing
tests against the `PreviewDetector`.
https://dart.googlesource.com/external/github.com/flutter/flutter/+/5959600e16c6b357cf3b66f502481f6cec16fad5

DEPS

diff --git a/DEPS b/DEPS
index 66a4736..9b5f68d 100644
--- a/DEPS
+++ b/DEPS
@@ -237,7 +237,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'a3bb94210c8da3001bfdec8e4207444f827e2b3c',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'e197ed2ac99965d003403fd0587c8664d48fd0dd',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '5959600e16c6b357cf3b66f502481f6cec16fad5',
 
   'flutter/engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index b8fccc1..230b95a 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"e197ed2ac99965d003403fd0587c8664d48fd0dd",
+  "flutter":"5959600e16c6b357cf3b66f502481f6cec16fad5",
   "flutter/engine/src/flutter/third_party/dart":"a3bb94210c8da3001bfdec8e4207444f827e2b3c"
 }
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