Experimental getDiagnostics request (#24480).

* Adds `getDiagnostics` request with *very* provisional diagnostics payload (primary goal here is to plumb this fully through and iterate).
* Updates codegenerator to skip doc generation for experimental APIs.

Additional context here: https://github.com/dart-lang/sdk/issues/24480.

BUG=24480
R=brianwilkerson@google.com, scheglov@google.com

Review URL: https://codereview.chromium.org/1416093007 .

pkg/analysis_server/tool/spec/generated/java/AnalysisServer.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, the Dart project authors.
+ * Copyright (c) 2015, the Dart project authors.
  *
  * Licensed under the Eclipse Public License v1.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
@@ -442,6 +442,13 @@ public interface AnalysisServer {
    */
   public void execution_setSubscriptions(List<String> subscriptions);
 
+  /**
+   * {@code experimental.getDiagnostics}
+   *
+   * Return server diagnostics.
+   */
+  public void experimental_getDiagnostics(GetDiagnosticsConsumer consumer);
+
   /**
    * Return {@code true} if the socket is open.
    */