Add information about existing imports to completion response.

This adds quite a lot of information though, because we send many
strings.

Before: 20686 characters.
After:  55345 characters.

Theoretically we could do better, and send information about existing
imports only once as well, so when the user continues typing in a
single file, without touching imports, we don't send any new data
about imports (maybe just a confirmation that it is still the same).
But I'm not sure if this is a worthwhile optimization.

Actually, even included suggestion sets have similar property - they
don't change for a given file, unless there are changes to other
libraries which we might want to include.

[email protected]

Change-Id: I2f55e2dc85508849146aa39eb279beabaec937c0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/103561
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov

pkg/analysis_server/doc/api.html

@@ -6,7 +6,7 @@
 // To regenerate the file, use the script
 // "pkg/analysis_server/tool/spec/generate_files".
 
-const String PROTOCOL_VERSION = '1.26.1';
+const String PROTOCOL_VERSION = '1.27.0';
 
 const String ANALYSIS_NOTIFICATION_ANALYZED_FILES = 'analysis.analyzedFiles';
 const String ANALYSIS_NOTIFICATION_ANALYZED_FILES_DIRECTORIES = 'directories';
@@ -116,6 +116,10 @@ const String COMPLETION_NOTIFICATION_AVAILABLE_SUGGESTIONS_CHANGED_LIBRARIES =
     'changedLibraries';
 const String COMPLETION_NOTIFICATION_AVAILABLE_SUGGESTIONS_REMOVED_LIBRARIES =
     'removedLibraries';
+const String COMPLETION_NOTIFICATION_EXISTING_IMPORTS =
+    'completion.existingImports';
+const String COMPLETION_NOTIFICATION_EXISTING_IMPORTS_FILE = 'file';
+const String COMPLETION_NOTIFICATION_EXISTING_IMPORTS_IMPORTS = 'imports';
 const String COMPLETION_NOTIFICATION_RESULTS = 'completion.results';
 const String COMPLETION_NOTIFICATION_RESULTS_ID = 'id';
 const String COMPLETION_NOTIFICATION_RESULTS_INCLUDED_ELEMENT_KINDS =
@@ -126,6 +130,7 @@ const String
 const String COMPLETION_NOTIFICATION_RESULTS_INCLUDED_SUGGESTION_SETS =
     'includedSuggestionSets';
 const String COMPLETION_NOTIFICATION_RESULTS_IS_LAST = 'isLast';
+const String COMPLETION_NOTIFICATION_RESULTS_LIBRARY_FILE = 'libraryFile';
 const String COMPLETION_NOTIFICATION_RESULTS_REPLACEMENT_LENGTH =
     'replacementLength';
 const String COMPLETION_NOTIFICATION_RESULTS_REPLACEMENT_OFFSET =