Pass the --preview-dart-2 flag to the analysis server.

Change-Id: I3b63a63ea26e09cd4e5b84c3933f0f4f6ddc2440
Reviewed-on: https://dart-review.googlesource.com/40465
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Devon Carew <devoncarew@google.com>

pkg/analysis_server/lib/src/analysis_server.dart

@@ -372,6 +372,7 @@ class AnalysisServer {
 
     defaultContextOptions.generateImplicitErrors = false;
     defaultContextOptions.useFastaParser = options.useCFE;
+    defaultContextOptions.previewDart2 = options.previewDart2;
 
     {
       String name = options.newAnalysisDriverLog;