Implement AnalysisDriver.getSourceKind() and use it in analyzer-cli.

pkg/analyzer_cli/lib/src/driver.dart

Index: pkg/analyzer_cli/lib/src/driver.dart
diff --git a/pkg/analyzer_cli/lib/src/driver.dart b/pkg/analyzer_cli/lib/src/driver.dart
index e77699f4a59af8310ec94a5f9fb3d18c98cfaba5..49ba67fce572f47b57dabab3b1f3fc7a39944010 100644
--- a/pkg/analyzer_cli/lib/src/driver.dart
+++ b/pkg/analyzer_cli/lib/src/driver.dart
@@ -230,7 +230,10 @@ class Driver implements CommandLineStarter {
     var libUris = <Uri>[];
     var parts = <Source>[];
     for (Source source in sourcesToAnalyze) {
-      if (context.computeKindOf(source) == SourceKind.PART) {
+      SourceKind sourceKind = analysisDriver != null
+          ? await analysisDriver.getSourceKind(source.fullName)
+          : context.computeKindOf(source);
+      if (sourceKind == SourceKind.PART) {
         parts.add(source);
         continue;
       }