Issue 23027. Try to make priority files analyzable.

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index fc1957cd7f3710d4afa6c054b38430fa0b40e142..80452d362abae73e4e338cc21e052a76feca0d97 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -463,6 +463,14 @@ class AnalysisServer {
         return new ContextSourcePair(context, source);
       }
     }
+    // try to find a context for which the file is a priority source
+    for (InternalAnalysisContext context in folderMap.values) {
+      List<Source> sources = context.getSourcesWithFullName(path);
+      if (sources.isNotEmpty) {
+        Source source = sources.first;
+        return new ContextSourcePair(context, source);
+      }
+    }
     // file-based source
     Source fileSource = file != null ? file.createSource() : null;
     return new ContextSourcePair(null, fileSource);
@@ -916,10 +924,27 @@ class AnalysisServer {
         new HashMap<AnalysisContext, List<Source>>();
     List<String> unanalyzed = new List<String>();
     Source firstSource = null;
-    files.forEach((file) {
+    files.forEach((String file) {
       ContextSourcePair contextSource = getContextSourcePair(file);
       AnalysisContext preferredContext = contextSource.context;
       Source source = contextSource.source;
+      // Try to make the file analyzable.
+      // If it is not in any context yet, add it to the first one which
+      // could use it, e.g. imports its package, even if not the library.
+      if (preferredContext == null) {
+        Resource resource = resourceProvider.getResource(file);
+        if (resource is File && resource.exists) {
+          for (AnalysisContext context in folderMap.values) {
+            Uri uri = context.sourceFactory.restoreUri(source);
+            if (uri.scheme != 'file') {
+              preferredContext = context;
+              source = ContextManager.createSourceInContext(context, resource);
+              break;
+            }
+          }
+        }
+      }
+      // Fill the source map.
       bool contextFound = false;
       for (AnalysisContext context in folderMap.values) {
         if (context == preferredContext ||